use strict;
use warnings;

open (FILE, "<test.data") or die "open: $!";
my @file_contents = <FILE>;
close (FILE);

@file_contents = map {s/stefan/kabel/g; $_;} @file_contents;

open (FILE, ">test.data") or die "open 2: $!";
print FILE $_ foreach (@file_contents);
close (FILE);