use strict;
my $folder=".";
my $file_mask="\.txt";
my $file;
opendir(DIR, "$folder");
my @files =grep(/$file_mask$/,readdir(DIR));
closedir(DIR);
foreach $file (@files) {
	print "$file\n";
	# Finds a file called 1txt (WHY)
}

opendir(DIR, "$folder");
@files =grep(/\.txt$/,readdir(DIR));
closedir(DIR);
foreach $file (@files) {
	print "$file\n";
	# Does not find the file called 1txt (As expected)
}