opendir my $DIR, '.' or die "Can't opendir . : $!\n";
my %single;
my %other = (did => 'mrg', mrg => 'did');

while (my $file = readdir $DIR) {
#print "$file\n";
	my ($name,$ext) = ($file =~ /^(.*)\.(did|mrg)$/)
		or next;
	if (exists $single{"$name.$other{$ext}"}) {
		delete $single{"$name.$other{$ext}"};
	} else {
		$single{$file} = undef;
	}
}

print "Unpaired files:\n";
foreach my $file (keys %single) {
	print "\t$file\n";
}