use strict;
use warnings;
# load your FTP module
use Tie::File;


# define @files
my @files = (
  ...
);

# set up FTP
...


my %UMLAUTE = (
  'Ä'=>'Ä',
  'Ö'=>'Ö',
  'Ü'=>'Ü',
  'ä'=>'ä',
  'ö'=>'ö',
  'ü'=>'ü',
  'ß'=>'ß',
);

my $RE_UMLAUTE = join("|", keys(%UMLAUTE));

for my $file (@files) {   

    tie my @lines, 'Tie::File', $file or die "Could not tie $file: $!";

    for my $line ( @lines ) {
        $line =~ s/($RE_UMLAUTE)/$UMLAUTE{$1}/g;
    }

    untie @lines;

    copy($datei,"C:\\xampp\\htdocs\\aserver\\AAA\\AAApflugi\\listen\\".$datei); 

    $ftp->ascii();
    $ftp->put("$datei");
}