while (<$in>) {
    my @tmp = split ',';             # perldoc -f split
    my ($number, $name) = @tmp[0,2]; # perldata, Stichwort 'slice'
    next if ! defined $name;
    push @{$dump_packs{$name}}, $number ; # key: name, value: number array (!)
}

for my $key (sort keys %dump_packs) { # write the name and occurency in a new file

#-> @pack = $key; # FALSCH: @pack enthält nur den Wert $key
# besser:
    my @pack = @{$dump_packs{$key}};
    
#-> open(my $out, '>', "$resdir/ ${dumpfile}_dump.txt") or die $!; 
#                               ^ Leerzeichen im Pfad 
    # File im Append-Modus (!) öffnen
    open(my $out, '>>', "$resdir/${dumpfile}_dump.txt") or die $!; 

#-> print $out join('', $key); # key and occurency ???
    print $out $key . "," . scalar(@pack) . "\n"; # Name, Anzahl 
    close $out;
}