my @sorted = sort by_year_and_month @files;

sub by_year_and_month {
	year($a) <=> year($b) or month($a) <=>month($b);
}

sub month {
	substr shift, 0, 2; #used to say '0, 1' but chipmunk caught the mistake
}

sub year 
	substr shift, 2, 2; #used to be 2, 3 but see above!
}