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Frequently asked questions about the PERL programming language.
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Contents of the PERL_FAQ.TXT file

Most Frequently Asked Questions

This list is maintained by Tom Christiansen, and is archived on
[] in the file pub/perl/info/faq. If you have any suggested
additions or corrections to this article, please send them to Tom at
either or . Special thanks to Larry
Wall for initially reviewing this list for accuracy and especially for
writing and releasing Perl in the first place.

List of Questions:

1) What is Perl?
2) Where can I get Perl?
3) How can I get Perl via UUCP?
4) Where can I get more documentation and examples for Perl?
5) Are archives of comp.lang.perl available?
6) How do I get Perl to run on machine FOO?
7) What are all these [email protected]%<> signs and how do I know when to use them?
8) Why don't backticks work as they do in shells?
9) How come Perl operators have different precedence than C operators?
10) How come my converted awk/sed/sh script runs more slowly in Perl?
11) There's an a2p and an s2p; why isn't there a p2c (perl-to-C)?
12) Where can I get undump for my machine?
13) How can I call my system's unique C functions from Perl?
14) Where do I get the include files to do ioctl() or syscall()?
15) Why doesn't "local($foo) = ;" work right?
16) How can I detect keyboard input without reading it?
17) How can I make an array of arrays or other recursive data types?
18) How can I quote a variable to use in a regexp?
19) Why do setuid Perl scripts complain about kernel problems?
20) How do I open a pipe both to and from a command?
21) How can I change the first N letters of a string?
22) How can I manipulate fixed-record-length files?
23) How can I make a file handle local to a subroutine?
24) How can I extract just the unique elements of an array?
25) How can I call alarm() from Perl?
26) How can I test whether an array contains a certain element?
27) How can I do an atexit() or setjmp()/longjmp() in Perl?
28) Why doesn't Perl interpret my octal data octally?
29) Where can I get a perl-mode for emacs?
30) How can I use Perl interactively?
31) How do I sort an associative array by value instead of by key?
32) How can I capture STDERR from an external command?
33) Why doesn't open return an error when a pipe open fails?
34) How can I use curses with perl?

To skip ahead to a particular question, such as question 17, you can
search for the regular expression "^17)". Most pagers (more or less)
do this with the command /^17) followed by a carriage return.

1) What is Perl?

A programming language, by Larry Wall .

Here's the beginning of the description from the man page:

Perl is an interpreted language optimized for scanning arbitrary text
files, extracting information from those text files, and printing reports
based on that information. It's also a good language for many system
management tasks. The language is intended to be practical (easy to use,
efficient, complete) rather than beautiful (tiny, elegant, minimal). It
combines (in the author's opinion, anyway) some of the best features of C,
sed, awk, and sh, so people familiar with those languages should have
little difficulty with it. (Language historians will also note some
vestiges of csh, Pascal, and even BASIC-PLUS.) Expression syntax
corresponds quite closely to C expression syntax. Unlike most Unix
utilities, Perl does not arbitrarily limit the size of your data--if
you've got the memory, Perl can slurp in your whole file as a single
string. Recursion is of unlimited depth. And the hash tables used by
associative arrays grow as necessary to prevent degraded performance.
Perl uses sophisticated pattern matching techniques to scan large amounts
of data very quickly. Although optimized for scanning text, Perl can also
deal with binary data, and can make dbm files look like associative arrays
(where dbm is available). Setuid Perl scripts are safer than C programs
through a dataflow tracing mechanism which prevents many stupid security
holes. If you have a problem that would ordinarily use sed or awk or sh,
but it exceeds their capabilities or must run a little faster, and you
don't want to write the silly thing in C, then Perl may be for you. There
are also translators to turn your sed and awk scripts into Perl scripts.

2) Where can I get Perl?

From any comp.sources.misc archive. Initial sources were posted to
Volume 18, Issues 19-54 at patchlevel 3. The Patches 4-10 were posted
to Volume 20, Issues 56-62.

These machines, at the very least, definitely have it available for
anonymous FTP:

If you are in Europe, you might using the following site. This
information thanks to "Henk P. Penning" :

FTP: Perl stuff is in the PERL directory on (

Email: Send a message to '[email protected]' containing:
path your_email_address
send help
The path-line may be omitted if your message contains a normal From:-line.
You will receive a help-file and an index of the directory that contains
the Perl stuff.

3) How can I get Perl via UUCP?

You can get it from the site osu-cis; here is the appropriate info,
thanks to J Greely or .

E-mail contact:
Get these two files first:
Current Perl distribution:
osu-cis!~/perl/4.0/[email protected]/perl.kitXX.Z (XX=01-37)
How to reach osu-cis via uucp(L.sys/Systems file lines):
# Direct Trailblazer
osu-cis Any ACU 19200 1-614-292-5112 in:--in:--in: Uanon
# Direct V.32 (MNP 4)
# dead, dead, dead...sigh.
#osu-cis Any ACU 9600 1-614-292-1153 in:--in:--in: Uanon
# Micom port selector, at 1200, 2400, or 9600 bps.
# Replace ##'s below with 12, 24, or 96 (both speed and phone number).
osu-cis Any ACU ##00 1-614-292-31## "" \r\c Name? osu-cis nected \c GO \d\r\ d\r\d\r in:--in:--in:

Modify as appropriate for your site, of course, to deal with your
local telephone system. There are no limitations concerning the hours
of the day you may call.

Another possiblity is to use UUNET, although they charge you
for it. You have been duly warned. Here's the advert:

Anonymous Access to UUNET's Source Archives


UUNET now provides access to its extensive collection of UNIX
related sources to non- subscribers. By calling 1-900-468-7727
and using the login "uucp" with no password, anyone may uucp any
of UUNET's on line source collection. Callers will be charged 40
cents per minute. The charges will appear on their next tele-
phone bill.

The file uunet!~/help contains instructions. The file
uunet!~/ls-lR.Z contains a complete list of the files available
and is updated daily. Files ending in Z need to be uncompressed
before being used. The file uunet!~/compress.tar is a tar
archive containing the C sources for the uncompress program.

This service provides a cost effective way of obtaining
current releases of sources without having to maintain accounts
with UUNET or some other service. All modems connected to the
900 number are Telebit T2500 modems. These modems support all
standard modem speeds including PEP, V.32 (9600), V.22bis (2400),
Bell 212a (1200), and Bell 103 (300). Using PEP or V.32, a 1.5
megabyte file such as the GNU C compiler would cost $10 in con-
nect charges. The entire 55 megabyte X Window system V11 R4
would cost only $370 in connect time. These costs are less than
the official tape distribution fees and they are available now
via modem.

UUNET Communications Services
3110 Fairview Park Drive, Suite 570
Falls Church, VA 22042
+1 703 876 5050 (voice)
+1 703 876 5059 (fax)
[email protected]

4) Where can I get more documentation and examples for Perl?

If you've been dismayed by the ~75-page Perl man page (or is that man
treatise?) you should look to ``the Camel Book'', written by Larry and
Randal L. Schwartz , published as a Nutshell
Handbook by O'Reilly & Associates and entitled _Programming Perl_.
Besides serving as a reference guide for Perl, it also contains
tutorial material, is a great source of examples and cookbook
procedures, as well as wit and wisdom, tricks and traps, pranks and
pitfalls. The code examples contained therein are available via
anonymous FTP from in nutshell/perl/perl.tar.Z for your
retrieval. Corrections and additions to the book can be found
in the Perl man page right before the BUGS section under the

If you can't find the book in your local technical bookstore, the book
may be ordered directly from O'Reilly by calling 1-800-dev-nuts if in
North America (that's 1-800-338-6887 for those poor folks without
handy mnemonic numbers on their phones) and 1-707-829-0515.
Autographed copies are available from TECHbooks by calling
1-503-646-8257 or mailing [email protected] Cost is ~30$US for the
regular version, 40$US for the special autographed one.

The book's ISBN is 0-937175-64-1.

For other examples of Perl scripts, look in the Perl source directory in
the eg subdirectory. You can also find a good deal of them on in the pub/perl/scripts/ subdirectory.

Another source for examples, currently only for anonymous FTP, is on []. This contains, amongst other things,
a copy of the newsgroup up through Aug 91, a text retrieval database
for the newsgroup, a rather old and short troff version of Tom Christiansen' s
perl tutorial (this was the version presented at Washington DC USENIX),
and quite a few of Tom's scripts. You can look at the INDEX file
in /pub/perl/INDEX for a list of what's in that directory. In the
future, monthly updates of all the newsgroup's articles will be
placed there, and the by-subject indexing into subfolders will be

Larry Wall has published a 3-part article on perl in Unix World
(August through October of 1991), and Rob Kolstad also had a
3-parter in Unix Review (May through July of 1990).

A nice reference guide by Johan Vromans is also available;
It is distributed in LaTeX (source) and PostScript (ready to
print) forms. Obsolete versions may still be available in TeX and troff
forms, although these don't print as nicely. The official kit
includes both LaTeX and PostScript forms, and can be FTP'd from [], file DOC/perlref-
The reference guide comes with the O'Reilly book in a nice, glossy
card format.

Additionally, USENIX and SUG have been sponsoring tutorials of varying
lengths on Perl at their system administration and general
conferences, taught by Tom Christiansen and/or
Rob Kolstad ; you might consider attending one of
these. Special cameo appearances by these folks may also be
negotiated; send us mail if your organization is interested in having
a Perl class taught.

You should definitely read the USENET comp.lang.perl newsgroup for all
sorts of discussions regarding the language, bugs, features, history,
humor, and trivia. In this respect, it functions both as a comp.lang.*
style newsgroup and also as a user group for the language; in fact,
there's a mailing list called ``perl-users'' that is bidirectionally
gatewayed to the newsgroup. Larry Wall is a very frequent poster here, as
well as many (if not most) of the other seasoned Perl programmers. It's
the best place for the very latest information on Perl.

5) Are archives of comp.lang.perl available?

Yes, although they're poorly organized. You can get them from
the host ( in the directory
/pub/comp.lang.perl. Perhaps by next month you'll be able to
get them from uunet as well. It contains these things:

comp.lang.perl.tar.Z -- the 5M tarchive in MH/news format
archives/ -- the unpacked 5M tarchive
unviewed/ -- new comp.lang.perl messages since 4-Feb or 5-Feb.

These are currently stored in news- or MH-style format; there are
subdirectories named things like "arrays", "programs", "taint", and
"emacs". Unfortunately, only the first ~1600 or so messages have been
so categorized, and we're now up to almost 5000. Furthermore, even
this categorization was haphazardly done and contains errors.

A more sophisticated query and retrieval mechanism is desirable.
Preferably one that allows you to retrieve article using a fast-access
indices, keyed on at least author, date, subject, thread (as in "trn")
and probably keywords. Right now, the MH pick command works for this,
but it is very slow to select on 5000 articles.

If you're serious about this, your best bet is probably to retrieve
the compressed tarchive and play with what you get. Any suggestions
how to better sort this all out are extremely welcome.

6) How do I get Perl to run on machine FOO?

Perl comes with an elaborate auto-configuration script that allows Perl
to be painlessly ported to a wide variety of platforms, including many
non-UNIX ones. Amiga and MS-DOS binaries are available on jpl-devvax for
anonymous FTP. Try to bring Perl up on your machine, and if you have
problems, examine the README file carefully, and if all else fails,
post to comp.lang.perl; probably someone out there has run into your
problem and will be able to help you.

7) What are all these [email protected]*%<> signs and how do I know when to use them?

Those are type specifiers: $ for scalar values, @ for indexed arrays,
and % for hashed arrays. The * means all types of that symbol name
and are sometimes used like pointers; the <> are used for inputting a
record from a filehandle. See question 17 for more on pointers.

Always make sure to use a $ for single values and @ for multiple ones.
Thus element 2 of the @foo array is accessed as $foo[2], not @foo[2],
which is a list of length one (not a scalar), and is a fairly common
novice mistake. Sometimes you can get by with @foo[2], but it's
not really doing what you think it's doing for the reason you think
it's doing it, which means one of these days, you'll shoot yourself
in the foot. Just always say $foo[2] and you'll be happier.

This may seem confusing, but try to think of it this way: you use the
character of the type which you *want back*. You could use @foo[1..3] for
a slice of three elements of @foo, or even @foo{'a','b',c'} for a slice of
of %foo. This is the same as using ($foo[1], $foo[2], $foo[3]) and
($foo{'a'}, $foo{'b'}, $foo{'c'}) respectively. In fact, you can even use
lists to subscript arrays and pull out more lists, like @foo[@bar] or
@foo{@bar}, where @bar is in both cases presumably a list of subscripts.

While there are a few places where you don't actually need these type
specifiers, except for files, you should always use them. Note that
is NOT the type specifier for files; it's the equivalent of awk's
getline function, that is, it reads a line from the handle FILE. When
doing open, close, and other operations besides the getline function on
files, do NOT use the brackets.

Beware of saying:
$foo = BAR;
Which wil be interpreted as
$foo = 'BAR';
and not as
$foo = ;
If you always quote your strings, you'll avoid this trap.

Normally, files are manipulated something like this (with appropriate
error checking added if it were production code):

open (FILE, ">/tmp/foo.$$"); print FILE "string\n"; close FILE;

If instead of a filehandle, you use a normal scalar variable with file
manipulation functions, this is considered an indirect reference to a
filehandle. For example,

$foo = "TEST01";
open($foo, "file");

After the open, these two while loops are equivalent:

while (<$foo>) {}
while () {}

as are these two statements:

close $foo;
close TEST01;

but NOT to this:

while (<$TEST01>) {} # error
^ note spurious dollar sign

This is another common novice mistake; often it's assumed that

open($foo, "output.$$");

will fill in the value of $foo, which was previously undefined.
This just isn't so -- you must set $foo to be the name of a valid
filehandle before you attempt to open it.

8) Why don't backticks work as they do in shells?

Because backticks do not interpolate within double quotes
in Perl as they do in shells.

Let's look at two common mistakes:

1) $foo = "$bar is `wc $file`";

This should have been:

$foo = "$bar is " . `wc $file`;

But you'll have an extra newline you might not expect. This
does not work as expected:

2) $back = `pwd`; chdir($somewhere); chdir($back);

Because backticks do not automatically eat trailing or embedded
newlines. The chop() function will remove the last character from
a string. This should have been:

chop($back = `pwd`); chdir($somewhere); chdir($back);

You should also be aware that while in the shells, embedding
single quotes will protect variables, in Perl, you'll need
to escape the dollar signs.

Shell: foo=`cmd 'safe $dollar'`
Perl: $foo=`cmd 'safe \$dollar'`;

9) How come Perl operators have different precedence than C operators?

Actually, they don't; all C operators have the same precedence in Perl as
they do in C. The problem is with a class of functions called list
operators, e.g. print, chdir, exec, system, and so on. These are somewhat
bizarre in that they have different precedence depending on whether you
look on the left or right of them. Basically, they gobble up all things
on their right. For example,

unlink $foo, "bar", @names, "others";

will unlink all those file names. A common mistake is to write:

unlink "a_file" || die "snafu";

The problem is that this gets interpreted as

unlink("a_file" || die "snafu");

To avoid this problem, you can always make them look like function calls
or use an extra level of parentheses:

(unlink "a_file") || die "snafu";
unlink("a_file") || die "snafu";

See the Perl man page's section on Precedence for more gory details.

10) How come my converted awk/sed/sh script runs more slowly in Perl?

The natural way to program in those languages may not make for the fastest
Perl code. Notably, the awk-to-perl translator produces sub-optimal code;
see the a2p man page for tweaks you can make.

Two of Perl's strongest points are its associative arrays and its regular
expressions. They can dramatically speed up your code when applied
properly. Recasting your code to use them can help alot.

How complex are your regexps? Deeply nested sub-expressions with {n,m} or
* operators can take a very long time to compute. Don't use ()'s unless
you really need them. Anchor your string to the front if you can.

Something like this:
next unless /^.*%.*$/;
runs more slowly than the equivalent:
next unless /%/;

Note that this:
next if /Mon/;
next if /Tue/;
next if /Wed/;
next if /Thu/;
next if /Fri/;
runs faster than this:
next if /Mon/ || /Tue/ || /Wed/ || /Thu/ || /Fri/;
which in turn runs faster than this:
next if /Mon|Tue|Wed|Thu|Fri/;
which runs *much* faster than:
next if /(Mon|Tue|Wed|Thu|Fri)/;

There's no need to use /^.*foo.*$/ when /foo/ will do.

Remember that a printf costs more than a simple print.

Don't split() every line if you don't have to.

Another thing to look at is your loops. Are you iterating through
indexed arrays rather than just putting everything into a hashed
array? For example,

@list = ('abc', 'def', 'ghi', 'jkl', 'mno', 'pqr', 'stv');

for $i ($[ .. $#list) {
if ($pattern eq $list[$i]) { $found++; }

First of all, it would be faster to use Perl's foreach mechanism
instead of using subscripts:

foreach $elt (@list) {
if ($pattern eq $elt) { $found++; }

Better yet, this could be sped up dramatically by placing the whole
thing in an associative array like this:

%list = ('abc', 1, 'def', 1, 'ghi', 1, 'jkl', 1,
'mno', 1, 'pqr', 1, 'stv', 1 );
$found += $list{$pattern};

(but put the %list assignment outside of your input loop.)

You should also look at variables in regular expressions, which is
expensive. If the variable to be interpolated doesn't change over the
life of the process, use the /o modifier to tell Perl to compile the
regexp only once, like this:

for $i (1..100) {
if (/$foo/o) {
do some_func($i);

Finally, if you have a bunch of patterns in a list that you'd like to
compare against, instead of doing this:

@pats = ('_get.*', 'bogus', '_read', '.*exit', '_write');
foreach $pat (@pats) {
if ( $name =~ /^$pat$/ ) {
do some_fun();

If you build your code and then eval it, it will be much faster.
For example:

@pats = ('_get.*', 'bogus', '_read', '.*exit', '_write');
$code = < while () {
foreach $pat (@pats) {
$code .= < if ( /^$pat\$/ ) {
do some_fun();
$code .= "}\n";
print $code if $debugging;
eval $code;

11) There's an a2p and an s2p; why isn't there a p2c (perl-to-C)?

Because the Pascal people would be upset that we stole their name. ๐Ÿ™‚

The dynamic nature of Perl's do and eval operators (and remember that
constructs like s/$mac_donald/$mac_gregor/eieio count as an eval) would
make this very difficult. To fully support them, you would have to put
the whole Perl interpreter into each compiled version for those scripts
using them. This is what undump does right now, if your machine has it.
If what you're doing will be faster in C than in Perl, maybe it should
have been written in C in the first place. For things that ought to be
written in Perl, the interpreter will be just about as fast, because the
pattern matching routines won't work any faster linked into a C program.
Even in the case of simple Perl programs that don't do any fancy evals, the
major gain would be in compiling the control flow tests, with the rest
still being a maze of twisty, turny subroutine calls. Since these are not
usually the major bottleneck in the program, there's not as much to be
gained via compilation as one might think.

12) Where can I get undump for my machine?

The undump program comes from the TeX distribution. If you have TeX, then
you may have a working undump. If you don't, and you can't get one,
*AND* you have a GNU emacs working on your machine that can clone itself,
then you might try taking its unexec() function and compiling Perl with
-DUNEXEC, which will make Perl call unexec() instead of abort(). You'll
have to add unexec.o to the objects line in the Makefile. If you succeed,
post to comp.lang.perl about your experience so others can benefit from it.

13) How can I call my system's unique C functions from Perl?

If these are system calls and you have the syscall() function, then
you're probably in luck -- see the next question. For arbitrary
library functions, it's not quite so straight-forward. While you
can't have a C main and link in Perl routines, if you're
determined, you can extend Perl by linking in your own C routines.
See the usub/ subdirectory in the Perl distribution kit for an example
of doing this to build a Perl that understands curses functions. It's
neither particularly easy nor overly-documented, but it is feasible.

14) Where do I get the include files to do ioctl() or syscall()?

These are generated from your system's C include files using the h2ph
script (once called makelib) from the Perl source directory. This will
make files containing subroutine definitions, like &SYS_getitimer, which
you can use as arguments to your function.

You might also look at the h2pl subdirectory in the Perl source for how to
convert these to forms like $SYS_getitimer; there are both advantages and
disadvantages to this. Read the notes in that directory for details.

In both cases, you may well have to fiddle with it to make these work; it
depends how funny-looking your system's C include files happen to be.

If you're trying to get at C structures, then you might take a look
at using c2ph, which uses debugger "stab" entries generated
by your BSD or GNU C compiler to produce perl definitions for the
data structures. c2ph comes with the perl distribution.

15) Why doesn't "local($foo) = ;" work right?

Well, it does. The thing to remember is that local() provides an array
context, an that the syntax in an array context will read all the
lines in a file. To work around this, use:

$foo = ;

You can use the scalar() operator to cast the expression into a scalar

local($foo) = scalar();

16) How can I detect keyboard input without reading it?

You might check out the Frequently Asked Questions list in comp.unix.* for
things like this: the answer is essentially the same. It's very system
dependent. Here's one solution that works on BSD systems:

sub key_ready {
local($rin, $nfd);
vec($rin, fileno(STDIN), 1) = 1;
return $nfd = select($rin,undef,undef,0);

A closely related question is how to input a single character from the
keyboard. Again, this is a system dependent operation. The following
code that may or may not help you:

$BSD = -f '/vmunix';
if ($BSD) {
system "stty cbreak /dev/tty 2>&1";
else {
system "stty", 'cbreak',
system "stty", 'eol', '^A'; # note: real control A

$key = getc(STDIN);

if ($BSD) {
system "stty -cbreak /dev/tty 2>&1";
else {
system "stty", 'icanon';
system "stty", 'eol', '^@'; # ascii null
print "\n";

You could also handle the stty operations yourself for speed if you're
going to be doing a lot of them. This code works to toggle cbreak
and echo modes on a BSD system:

sub set_cbreak { # &set_cbreak(1) or &set_cbreak(0)
local($on) = $_[0];
require 'sys/';
$sgttyb_t = 'C4 S' unless $sgttyb_t;

ioctl(STDIN,$TIOCGETP,$sgttyb) || die "Can't ioctl TIOCGETP: $!";

@ary = unpack($sgttyb_t,$sgttyb);
if ($on) {
$ary[4] |= $CBREAK;
$ary[4] &= ~$ECHO;
} else {
$ary[4] &= ~$CBREAK;
$ary[4] |= $ECHO;
$sgttyb = pack($sgttyb_t,@ary);

ioctl(STDIN,&TIOCSETP,$sgttyb) || die "Can't ioctl TIOCSETP: $!";

Note that this is one of the few times you actually want to use the
getc() function; it's in general way too expensive to call for normal
I/O. Normally, you just use the syntax, or perhaps the read()
or sysread() functions.

17) How can I make an array of arrays or other recursive data types?

Remember that Perl isn't about nested data structures, but rather flat
ones, so if you're trying to do this, you may be going about it the
wrong way. You might try parallel arrays with common subscripts.

But if you're bound and determined, you can use the multi-dimensional
array emulation of $a{'x','y','z'}, or you can make an array of names
of arrays and eval it.

For example, if @name contains a list of names of arrays, you can
get at a the j-th element of the i-th array like so:

$ary = $name[$i];
$val = eval "\$$ary[$j]";

or in one line

$val = eval "\$$name[$i][\$j]";

You could also use the type-globbing syntax to make an array of *name
values, which will be more efficient than eval. Here @name hold
a list of pointers, which we'll have to dereference through a temporary

For example:

{ local(*ary) = $name[$i]; $val = $ary[$j]; }

In fact, you can use this method to make arbitrarily nested data
structures. You really have to want to do this kind of thing
badly to go this far, however, as it is notationally cumbersome.

Let's assume you just simply *have* to have an array of arrays of
arrays. What you do is make an array of pointers to arrays of
pointers, where pointers are *name values described above. You
initialize the outermost array normally, and then you build up your
pointers from there. For example:

@w = ( 'ww' .. 'xx' );
@x = ( 'xx' .. 'yy' );
@y = ( 'yy' .. 'zz' );
@z = ( 'zz' .. 'zzz' );

@ww = reverse @w;
@xx = reverse @x;
@yy = reverse @y;
@zz = reverse @z;

Now make a couple of array of pointers to these:

@A = ( *w, *x, *y, *z );
@B = ( *ww, *xx, *yy, *zz );

And finally make an array of pointers to these arrays:

@AAA = ( *A, *B );

To access an element, such as AAA[i][j][k], you must do this:

local(*foo) = $AAA[$i];
local(*bar) = $foo[$j];
$answer = $bar[$k];

Similar manipulations on associative arrays are also feasible.

You could take a look at package posted by Felix Lee
, which lets you simulate vectors and tables (lists and
associative arrays) by using type glob references and some pretty serious

In C, you're used to creating recursive datatypes for operations
like recursive decent parsing or tree traversal. In Perl, these
algorithms are best implemented using associative arrays. Take an
array called %parent, and build up pointers such that $parent{$person}
is the name of that person's parent. Make sure you remember that
$parent{'adam'} is 'adam'. ๐Ÿ™‚ With a little care, this approach can
be used to implement general graph traversal algorithms as well.

18) How can I quote a variable to use in a regexp?

From the manual:

$pattern =~ s/(\W)/\\$1/g;

Now you can freely use /$pattern/ without fear of any unexpected
meta-characters in it throwing off the search. If you don't know
whether a pattern is valid or not, enclose it in an eval to avoid
a fatal run-time error.

19) Why do setuid Perl scripts complain about kernel problems?

This message:


is triggered because setuid scripts are inherently insecure due to a
kernel bug. If your system has fixed this bug, you can compile Perl
so that it knows this. Otherwise, create a setuid C program that just
execs Perl with the full name of the script.

20) How do I open a pipe both to and from a command?

In general, this is a dangerous move because you can find yourself in a
deadlock situation. It's better to put one end of the pipe to a file.
For example:

# first write some_cmd's input into a_file, then
open(CMD, "some_cmd its_args < a_file |");
while () {

# or else the other way; run the cmd
open(CMD, "| some_cmd its_args > a_file");
while ($condition) {
print CMD "some output\n";
# other code deleted
close CMD || warn "cmd exited $?";

# now read the file
while () {

If you have ptys, you could arrange to run the command on a pty and
avoid the deadlock problem. See the package in the
distributed library for ways to do this.

At the risk of deadlock, it is theoretically possible to use a
fork, two pipe calls, and an exec to manually set up the two-way
pipe. (BSD system may use socketpair() in place of the two pipes,
but this is not as portable.)

Here's one example of this that assumes it's going to talk to
something like adb, both writing to it and reading from it. This
is presumably safe because you "know" that commands like adb will
read a line at a time and output a line at a time. Programs like
sort that read their entire input stream first, however, are quite
apt to cause deadlock.

Use this way:

require '';
$child = &open2(RDR,WTR,"some cmd to run and its args");

Unqualified filehandles will be interpreted in their caller's package,
although &open2 lives in its open package (to protect its state data).
It returns the child process's pid if successful, and generally
dies if unsuccessful. You may wish to change the dies to warnings,
or trap the call in an eval. You should also flush STDOUT before
calling this.

# &open2: tom christiansen,
# usage: $pid = &open2('rdr', 'wtr', 'some cmd and args');
# spawn the given $cmd and connect $rdr for
# reading and $wtr for writing. return pid
# of child, or 0 on failure.
# WARNING: this is dangerous, as you may block forever
# unless you are very careful.
# $wtr is left unbuffered.
# abort program if
# rdr or wtr are null
# pipe or fork or exec fails

package open2;
$fh = 'FHOPEN000'; # package static in case called more than once

sub main'open2 {
local($dad_rdr, $dad_wtr, $cmd) = @_;

$dad_rdr ne '' || die "open2: rdr should not be null";
$dad_wtr ne '' || die "open2: wtr should not be null";

# force unqualified filehandles into callers' package
local($package) = caller;
$dad_rdr =~ s/^[^']+$/$package'$&/;
$dad_wtr =~ s/^[^']+$/$package'$&/;

local($kid_rdr) = ++$fh;
local($kid_wtr) = ++$fh;

pipe($dad_rdr, $kid_wtr) || die "open2: pipe 1 failed: $!";
pipe($kid_rdr, $dad_wtr) || die "open2: pipe 2 failed: $!";

if (($kidpid = fork) < 0) {
die "open2: fork failed: $!";
} elsif ($kidpid == 0) {
close $dad_rdr; close $dad_wtr;
open(STDIN, ">&$kid_rdr");
open(STDOUT, ">&$kid_wtr");
print STDERR "execing $cmd\n";
exec $cmd;
die "open2: exec of $cmd failed";
close $kid_rdr; close $kid_wtr;
select((select($dad_wtr), $| = 1)[0]); # unbuffer pipe
1; # so require is happy

21) How can I change the first N letters of a string?

Remember that the substr() function produces an lvalue, that is, it may be
assigned to. Therefore, to change the first character to an S, you could
do this:

substr($var,0,1) = 'S';

This assumes that $[ is 0; for a library routine where you can't know $[,
you should use this instead:

substr($var,$[,1) = 'S';

While it would be slower, you could in this case use a substitute:

$var =~ s/^./S/;

But this won't work if the string is empty or its first character is a
newline, which "." will never match. So you could use this instead:

$var =~ s/^[^\0]?/S/;

To do things like translation of the first part of a string, use substr,
as in:

substr($var, $[, 10) =~ tr/a-z/A-Z/;

If you don't know then length of what to translate, something like
this works:

/^(\S+)/ && substr($_,$[,length($1)) =~ tr/a-z/A-Z/;

For some things it's convenient to use the /e switch of the
substitute operator:

s/^(\S+)/($tmp = $1) =~ tr#a-z#A-Z#, $tmp/e

although in this case, it runs more slowly than does the previous example.

22) How can I manipulate fixed-record-length files?

The most efficient way is using pack and unpack. This is faster than
using substr. Here is a sample chunk of code to break up and put back
together again some fixed-format input lines, in this case, from ps.

# sample input line:
# 15158 p5 T 0:00 perl /mnt/tchrist/scripts/now-what
$ps_t = 'A6 A4 A7 A5 A*';
open(PS, "ps|");
$_ = ; print;
while () {
($pid, $tt, $stat, $time, $command) = unpack($ps_t, $_);
for $var ('pid', 'tt', 'stat', 'time', 'command' ) {
print "$var: <", eval "\$$var", ">\n";
print 'line=', pack($ps_t, $pid, $tt, $stat, $time, $command), "\n" ;

23) How can I make a file handle local to a subroutine?

You use the type-globbing *VAR notation. Here is some code to cat an
include file, calling itself recursively on nested local include files
(i.e. those with #include "file", not #include ):

sub cat_include {
local($name) = @_;

warn "\n";
if (!open (FILE, $name)) {
warn "can't open $name: $!\n";
while () {
if (/^#\s*include "([^"]*)"/) {
} else {
close FILE;

24) How can I extract just the unique elements of an array?

There are several possible ways, depending on whether the
array is ordered and you wish to preserve the ordering.

a) If @in is sorted, and you want @out to be sorted:

$prev = 'nonesuch';
@out = grep($_ ne $prev && (($prev) = $_), @in);

This is nice in that it doesn't use much extra memory,
simulating uniq's behavior of removing only adjacent

b) If you don't know whether @in is sorted:

undef %saw;
@out = grep(!$saw{$_}++, @in);

c) Like (b), but @in contains only small integers:

@out = grep(!$saw[$_]++, @in);

d) A way to do (b) without any loops or greps:

undef %saw;
@saw{@in} = ();
@out = sort keys %saw; # remove sort if undesired

e) Like (d), but @in contains only small positive integers:

undef @ary;
@ary[@in] = @in;
@out = sort @ary;

25) How can I call alarm() from Perl?

It's available as a built-in as of version 3.038. If you
want finer granularity than 1 second and have itimers
and syscall() on your system, you can use this.

It takes a floating-point number representing how long
to delay until you get the SIGALRM, and returns a floating-
point number representing how much time was left in the
old timer, if any. Note that the C function uses integers,
but this one doesn't mind fractional numbers.

# alarm; send me a SIGALRM in this many seconds (fractions ok)
# tom christiansen
sub alarm {
local($ticks) = @_;
local($isecs, $iusecs, $secs, $usecs);

local($SYS_setitimer) = 83; # require
local($ITIMER_REAL) = 0; # require sys/
local($itimer_t) = 'L4'; # confirm with sys/time.h

$secs = int($ticks);
$usecs = ($ticks - $secs) * 1e6;

$out_timer = pack($itimer_t,0,0,0,0);
$in_timer = pack($itimer_t,0,0,$secs,$usecs);

syscall($SYS_setitimer, $ITIMER_REAL, $in_timer, $out_timer)
&& die "alarm: setitimer syscall failed: $!";

($isecs, $iusecs, $secs, $usecs) = unpack($itimer_t,$out_timer);
return $secs + ($usecs/1e6);

26) How can I test whether an array contains a certain element?

There are several ways to approach this. If you are going to make this
query many times and the values are arbitrary strings, the fastest way is
probably to invert the original array and keep an associative array around
whose keys are the first array's values.

@blues = ('turquoise', 'teal', 'lapis lazuli');
undef %is_blue;
grep ($is_blue{$_}++, @blues);

Now you can check whether $is_blue{$some_color}. It might have been a
good idea to keep the blues all in an assoc array in the first place.

If the values are all small integers, you could use a simple
indexed array. This kind of an array will take up less

@primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
undef @is_tiny_prime;
grep($is_tiny_prime[$_]++, @primes);

Now you check whether $is_tiny_prime[$some_number].

If the values in question are integers, but instead of strings,
you can save quite a lot of space by using bit strings instead:

@articles = ( 1..10, 150..2000, 2017 );
undef $read;
grep (vec($read,$_,1) = 1, @articles);

Now check whether vec($read,$n,1) is true for some $n.

27) How can I do an atexit() or setjmp()/longjmp() in Perl?

Perl's exception-handling mechanism is its eval operator. You
can use eval as setjmp, and die as longjmp. Here's an example
of Larry's for timed-out input, which in C is often implemented
using setjmp and longjmp:

sub TIMEOUT { die "restart input\n"; }

do {
eval '&realcode';
} while [email protected] =~ /^restart input/;

sub realcode {
alarm 15;
$ans = ;

Here's an example of Tom's for doing atexit() handling:

sub atexit { push(@_exit_subs, @_); }

sub _cleanup { unlink $tmp; }


eval <<'End_Of_Eval'; $here = __LINE__;
# as much code here as you want

$oops = [email protected]; # save error message

# now call his stuff
for (@_exit_subs) { do $_(); }

$oops && ($oops =~ s/\(eval\) line (\d+)/$0 .
" line " . ($1+$here)/e, die $oops);

You can register your own routines via the &atexit function now. You
might also want to use the &realcode method of Larry's rather than
embedding all your code in the here-is document. Make sure to leave
via die rather than exit, or write your own &exit routine and call
that instead. In general, it's better for nested routines to exit
via die rather than exit for just this reason.

Eval is also quite useful for testing for system dependent features,
like symlinks, or using a user-input regexp that might otherwise
blowup on you.

28) Why doesn't Perl interpret my octal data octally?

Perl only understands octal and hex numbers as such when they occur
as constants in your program. If they are read in from somewhere
and assigned, then no automatic conversion takes place. You must
explicitly use oct() or hex() if you want this kind of thing to happen.
Actually, oct() knows to interpret both hex and octal numbers, while
hex only converts hexadecimal ones. For example:

print "What mode would you like? ";
$mode = ;
$mode = oct($mode);
unless ($mode) {
print "You can't really want mode 0!\n";
chmod $mode, $file;

Without the octal conversion, a requested mode of 755 would turn
into 01363, yielding bizarre file permissions of --wxrw--wt.

If you want something that handles decimal, octal and hex input,
you could follow the suggestion in the man page and use:

$val = oct($val) if $val =~ /^0/;

29) Where can I get a perl-mode for emacs?

In the perl4.0 source directory, you'll find a directory called
"emacs", which contains several files that should help you.

30) How can I use Perl interactively?

The easiest way to do this is to run Perl under its debugger.
If you have no program to debug, you can invoke the debugger
on an `empty' program like this:

perl -de 0

Now you can type in any legal Perl code, and it will be immediately
evaluated. You can also examine the symbol table, check variable
values, and if you want to, set breakpoints and do the other things
you can do in a symbolic debugger.

31) How do I sort an associative array by value instead of by key?

You have to declare a sort subroutine to do this. Let's assume
you want an ASCII sort on the values of the associative array %ary.
You could do so this way:

foreach $key (sort by_value keys %ary) {
print $key, '=', $ary{$key}, "\n";
sub by_value { $ary{$a} cmp $ary{$b}; }

If you wanted a descending numeric sort, you could do this:

sub by_value { $ary{$b} <=> $ary{$a}; }

If you wanted a function that didn't have the array name hard-wired
into it, you could so this:

foreach $key (&sort_by_value(*ary)) {
print $key, '=', $ary{$key}, "\n";
sub sort_by_value {
local(*x) = @_;
sub _by_value { $x{$a} cmp $x{$b}; }
sort _by_value keys %x;

If you want neither an alphabetic nor a numeric sort, then you'll
have to code in your own logic instead of relying on the built-in
signed comparison operators "cmp" and "<=>".

Note that if you're sorting on just a part of the value, such as a
piece you might extract via split, unpack, pattern-matching, or
substr, then rather than performing that operation inside your sort
routine on each call to it, it is significantly more efficient to
build a parallel array of just those portions you're sorting on, sort
the indices of this parallel array, and then to subscript your original
array using the newly sorted indices. This method works on both
regular and associative arrays, since both @ary[@idx] and @ary{@idx}
make sense. See page 245 in the Camel Book on "Sorting an Array by a
Computable Field" for a simple example of this.

32) How can I capture STDERR from an external command?

There are three basic ways of running external commands:

system $cmd;
$output = `$cmd`;
open (PIPE, "cmd |");

In the first case, both STDOUT and STDERR will go the same place as
the script's versions of these, unless redirected. You can always put
them where you want them and then read them back when the system
returns. In the second and third case, you are reading the STDOUT
*only* of your command. If you would like to have merged STDOUT and
STDERR, you can use shell file-descriptor redirection to dup STDERR to

$output = `$cmd 2>&1`;
open (PIPE, "cmd 2>&1 |");

Another possibility is to run STDERR into a file and read the file
later, as in

$output = `$cmd 2>some_file`;
open (PIPE, "cmd 2>some_file |");

Here's a way to read from both of them and know which descriptor
you got each line from. The trick is to pipe only STDERR through
sed, which then marks each of its lines, and then sends that
back into a merged STDOUT/STDERR stream, from which your Perl program
then reads a line at a time:

open (CMD,
"3>&1 (cmd args 2>&1 1>&3 3>&- | sed 's/^/STDERR:/' 3>&-) 3>&- |");

while () {
if (s/^STDERR://) {
print "line from stderr: ", $_;
} else {
print "line from stdout: ", $_;

33) Why doesn't open return an error when a pipe open fails?

These statements:

open(TOPIPE, "|bogus_command") || die ...
open(FROMPIPE, "bogus_command|") || die ...

will not fail just for lack of the bogus_command. They'll only
fail if the fork to run them fails, which is seldom at best.

If you're writing to the TOPIPE, you'll get a SIGPIPE if the child
exits prematurely or doesn't run. If you are reading from the
FROMPIPE, you need to check the close() to see what happened.

If you want an answer sooner than pipe buffering might otherwise
afford you, you can do something like this:

$kid = open (PIPE, "bogus_command |"); # XXX: check defined($kid)
(kill 0, $kid) || die "bogus_command failed";

This works fine if bogus_command doesn't have shell metas in it, but
if it does, the shell may well not have exited before the kill 0. You
could always introduce a delay:

$kid = open (PIPE, "bogus_command sleep 1;
(kill 0, $kid) || die "bogus_command failed";

but this is sometimes undesirable, and in any event does not guarantee
correct behavior. But it seems slightly better than nothing.

Similar tricks can be played with writable pipes if you don't wish to
catch the SIGPIPE.

34) How can I use curses with perl?

One way is to build a curseperl binary by linking in your C curses
library as described in the usub subdirectory of the perl sources.
This requires a bit of work, but it will be reasonably fast since it's
all in C (assuming you consider curses reasonably fast. ๐Ÿ™‚ Programs
written using this method require the modified curseperl, not vanilla
perl, to run.

Another possiblity is to use Henk Penning's cterm package, a curses
emulation library written in perl. cterm is actually a separate
program with which you communicate via a pipe. It is available from [] via anonymous ftp in the directory
pub/PERL. You may also acquire the package via email in compressed,
uuencoded form by sending a message to [email protected]
containing these lines:

send PERL/cterm.shar.Z

See question #2 for more information on how to get retrieve
other items of interest from the mail server there.

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