use Tie::File;
use strict;
use warnings;

my $file = '/path/to/file.ini';
tie my @array,'Tie::File',$file or die $!;
foreach my $line(@array){
  if($line =~ /DEIN_MUSTER/){
    chomp $line;
    $line .= "Anhang\n";
  }
}
untie @array;