my $stmt = "SELECT filename FROM tabelle";
my $sth = $dbh->prepare($stmt) or die $dbh->errstr();
$sth->execute() or die $dbh->errstr();

while(my ($text) = $sth->fetchrow_array()){
    print get_MD5($text);
}

sub get_MD5 {
    my ($text) = @_;
    my $md5=Digest::MD5->new;
    $md5->add($text);
    return $md5->hexdigest();
}