Using a module, you can run a function defined in that module. The module must be loaded at compile time.

However, Perl allows you to define subs in other files and load them in several ways. I'll show you some.

Here's the main file

#!/usr/bin/perl -w
use strict;

my $method = shift || 1;
my $filename = shift || 'other.pl';
my $sub = shift || 'hello';

if ($method == 1) {
    print "First method\n";
    {
    local $/;
    open OTHER, "< $filename" or die "can't open\n";
    my $other = <OTHER>;
    close OTHER;
    eval $other . ";$sub";
    }
}
elsif ($method == 2) {
    print "Second method\n";
    scalar eval `cat $filename`;
    eval $sub;
}
else {
    print "Third method\n";
    do $filename;
    eval $sub;
}
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In this file we define one sub.

#cat other.pl

sub hello {
    print "hello world\n";
}
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And one more in this other file.

# cat other2.pl
sub hi {
    print "hi, world!\n";
}
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As an example, you can run this script

$ perl test_runtime.pl 1 other.pl hello
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Where "1" is the method to use, "other.pl" is the file containing your function, "hello" is the function name.

Try also

$ perl test_runtime.pl 2 other2.pl hi
$ perl test_runtime.pl 3 other2.pl hi
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And see for yourself what happens.

CAVEAT. Using eval you are compiling and running code at run time. When you run a normal script, the Perl compiler will catch the mistakes and inform you about them. With eval, you should catch the errors by checking eval's return value. See the docs for more info.

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