#! /usr/bin/perl
use warnings;
use strict;

my $infile = $ARGV[0]; # 1. parameter auslesen
-f $infile or die "Error: '$infile' is not a file\n";

open (IN, $infile) or die "Error: couldn't read '$infile': $!\n";

while (<IN>) { # zeile fuer zeile einlesen => $_
  $_ =~s/ü/&ü/g;
  # .... usw.
  print $_;
} # while
close (IN);