Because I don't know how to do it. How can a piece of code understand if there already exists a connection to an external program so that it can re-use it without opening a new one thus saving time?

our $ALREADY_CONNECTED = 0; # this var's purpose is called a semaphore

DOCUMENT:
for my $document (@collection) {
    if (!$ALREADY_CONNECTED) {
        open2(...); # and save the handles for later usage
        $ALREADY_CONNECTED = 1; # whoa! magic!
        redo DOCUMENT;
    } else {
        # blah blah process $document here,
        # the in/out handles exist.
    };
};
[download]

Am I looking for something like PHP's require_once?

From your post the calling semantics are not clear to me. "My perl file is executed for every document in a large collection" - how is this perl file invoked? from a shell, from perl itself? If you are invoking perl anew for each file in the set (and exiting perl when done) you won't gain much with require.

If you showed some code it would be easier to help you. See How (Not) To Ask A Question.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

Thank you shmem.

I am to run this perl file from shell for every file in a large corpus (I have to, because this is only a small part of a large project. Execution continues with various other languages and scripts and so on afterwards), not from another perl file. So daxim's highly sophisticated code is unfortunately useless to me.

As you said, "require" would not bring me much advantage because upon finishing execution, even if I do not close() the connection, it will be auto-closed. And while processing the next file, same perl code will be executed and also "require"d file will be executed again.

Roughly, I want to have the following:
db.pl

open2 (\*INP, \*OUTP, 'path_to_external_program');
print "included";
1;
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maincode.pl

require "db.pl";
$file = $ARGV[0] #the next file to process
print OUTP some_command_using_$file; #exec some cmds on external prog
#more lines follow afterwards
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I want my maincode.pl to function for every file, as well as db.pl to print "included" only once. So that for the whole collection of thousands of documents, I will have only single connection throughout the batch execution. Possible in a way?

Possible in a way?

OF course yes! That's why I asked about calling semantics. Which part is gathering the collection of files to iterate over? the shell script? do you want perl to gather the files? what system are you on?

<handwaving style="amount: lots"> If you are gathering the file via a shell script, you could do something along these lines

( while read command param; do  # whatever method
    files=`command param`       # to gather the files
    for file in $files; do      # process files
        echo $file
    done
done) | maincode.pl
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and in maincode.pl

#!/usr/bin/perl
use IPC::Open2;
my $cmd = 'whatever'; # really, I have no idea what you are doing
$pid = open2(\*CHLD_OUT, \*CHLD_IN, $cmd) or die "oops: $!\n";

while(<STDIN>) {
    chomp;
    my $file = $_; # now do whatever with $file.
    open(I,'<', $file);
    while (my $line = <I>) {
        ... # do whatever with each line in the file
    }
}
[download]

but since I still don't know what you're up to, I can't give proper advice. See I know what I mean. Why don't you?

Maybe you really want some client/server stuff, or the perl code to act as a stream filter, and dispatch it's output to somewhere else. Each usage has different semantics; how can I know which are required, without a bit more of explanation from you?

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}