#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my %lut;
my $cur;
my %rev_lut;

while(<DATA>) {
   chomp;
   $cur = $1 if /(Gruppe\d+)/;
   push @{$lut{$cur}}, $_ unless (/Gruppe/ or /^\s*$/);
}

print Dumper \%lut;

while (my ($k, $v) = each %lut) {
   push @{$rev_lut{$_}}, $k for @$v;
}

print Dumper \%rev_lut;

_ _DATA_ _
Gruppe1
PC1
PC2
PC3

Gruppe2
PC2
PC4
PC5

Gruppe3
PC1
PC6