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

my %dirs = (
   './space/dir/dirname1' => 'Wert1',
   './space/dir/dirname2' => 'Wert2',
   './space/dir/dirname3' => 'Wert3',
   './space/dir/dirname4' => 'Wert4',
);

my %other_dirs = (
   dirname1 => 'dirname4',
   dirname2 => 'dirname5',
   dirname3 => 'dirname99',
   dirname4 => 'dirname100'
);

foreach (keys %dirs) {
   # matched bis zum letzten Slash, ein Slash muss enthalten sein
   next unless /^(.+)\/(.+)$/ && exists $other_dirs{$2};
   function1($_,$dirs{$_},"$1/$other_dirs{$2}",$other_dirs{$2});
}