Depending on how much interaction you want and what plug-ins / add-ins you want and provide, either Module::Pluggable, possibly together with NEXT or a really simple mixin-based approach might work for you. The following code shows how several files can create code in the same namespace:

package AddIns;
use strict;

our %functions;
our $AUTOLOAD;

sub register {
  my ($class,$key, $sub) = @_;
  $functions{$key} ||= [];
  push @{ $functions{$key} }, $sub;
};

sub AUTOLOAD {
  my (@args) = @_;
  $AUTOLOAD =~ m!.*::([^:]+)$!
    or die "Strange subroutine '$AUTOLOAD' called.";
  my $key = $1;
  die "Unknown slot '$key' called"
    unless exists $functions{$key};
  for my $sub (@{ $functions{$key} }) {
    eval { $sub->(@args); };
    if ($@) {
      warn "Error while calling slot '$key' : $@";
    };
  };
};

package AddIn::First;
AddIn->register('runonce', \&runonce);

sub runonce { print "myfirstaddin" };

package AddIn::Second;
AddIn->register('runonce', \&runonce);

sub runonce { print "mysecondaddin" };

package main;

AddIn->runonce;

# All of this is untested though
[download]