#!/usr/bin/perl

use strict;
use warnings;

my $pattern = qr/^((?<k>katze)|(?<h>hund)):(?<n>[^,]+),(?(<h>)(?<g>\d+),)(?<x>w|m)/;

blub("hund:waldi,23,m");
blub("katze:rosa,w");

sub blub {
    my $string = shift;
    print "-"x20;
    print "\nString:\t\t$string\n";
    if ($string =~ $pattern) {
        print "Tier:\t\t";
        if (defined($+{k})) {
            print "$+{k}\n";
        } elsif (defined($+{h})) {
            print "$+{h}\n";
        }
        print "Name:\t\t$+{n}\n" if (defined($+{n}));
        print "Gewicht:\t$+{g}kg\n" if (defined($+{g}));
        print "Geschlecht:\t$+{x}\n" if (defined($+{x}));
    } else {
        print "no matches...\n";
    }
}

$ ./test.pl
--------------------
String:         hund:waldi,23,m
Tier:           hund
Name:           waldi
Gewicht:        23kg
Geschlecht:     m
--------------------
String:         katze:rosa,w
Tier:           katze
Name:           rosa
Geschlecht:     w