#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;
use Carp;

my @txt = (
    "cn=Gruppenname,ou=Mail,ou=b,o=c",
    "cn=abcd,ou=a,ou=b,o=C;cn=efgh,ou=a,ou=b,o=c",
);

for my $line (@txt) {
    my @cn = $line =~ /cn=(\w+),/g;
    print Dumper \@cn;
}