The simplest way is to put the functions in a file, and require the file:

foo1.pl

use strict;
use warnings;

sub foo {
  print "This is the foo() function\n";
}

1;

And you can call foo() in the same way that prog1.pl does:

prog1.pl

#!/usr/bin/perl -w
# simple program to call foo() defined in foo1.pl
use strict;
require "foo1.pl";

foo();

This has some limitations: