my $dir = 'd:\old_log';
my @all_files = get_all_files($dir);

foreach my $file (@all_files) {
	print $file."\n"; 
	open my $FH, "<", "$dir\\".$file or die $!;
	my @zeilen = <$FH>;
	foreach my $line(@zeilen){
		# Hier müsste ich die Zeichen formatieren	
	}
	close($FH);
}

sub get_all_files {
	my $dir = shift;
	my $FH = undef;
	opendir $FH, $dir;
	my @all_files = grep{$_ =~ '.txt'} readdir($FH);
	closedir($FH);
	
	return @all_files;	
}