There are a few options, you have to decide what works for you.

First, build a library file, we'll call it functions.pl, it may look like this:

sub add {
  my( $x, $y ) = @_;
  return $x + $y;
}

sub subtract {
  my( $x, $y ) = @_;
  return $x - $y;
}

1;

Now, your program may look like this:

#!/path/to/perl -w
use strict;
$|++;

requre '/path/to/functions.pl';

print subtract( add( 3, 2 ), 1 );

Next, to build a Perl Module that @EXPORTs the functions:

package Functions;

use strict;
use Exporter;
use vars qw/@EXPORT @ISA/;

@ISA    = qw/Exporter/;
@EXPORT = qw/add subtract/;

BEGIN {
  import Functions @EXPORT;
}

sub add {
  return $_[0] + $_[1];
}

sub subtract {
  return $_[0] - $_[1];
}

1;

Our program may resemble this:

#!/usr/local/bin/perl -w
use strict;
$|++;

use lib '/path/to';
use Functions;

print subtract( add( 3, 2 ), 1 );

An object oriented version of the Perl Module:

package Functions;
use strict;
sub new { bless {}, shift }
sub add { shift; $_[0] + $_[1] }
sub subtract { shift; $_[0] - $_[1] }
1;

And our program:

#!/path/to/perl -w
use strict;
$|++;

use lib '/path/to';
use Functions;

my $f = Functions->new;

print $f->subtract( $f->add( 3, 2 ), 1 );

I would probably (in most cases) go with the OO Module, but you have to choose your path.

Enjoy!

--
Casey