Your sort call just reorders your list, it does nothing to "filter" the contents. You want to take the first last item off the generated list.

Also it doesn't make much sense to use s/// to modify the string to M-D-Y to then return a different string Y-M-D. Better would be to either use just a match m// and return the new string, or to actually switch things around with the s/// and return the modified string.

## Either
$mdy =~ m{^(...blahblah...)};
return "$3-$1-$2";

## Or . . .
$mdy =~ s{^(...blahblah...)}{$3-$1-$2};
return $mdy;

## Or maybe (with a new enough perl)
return $mdy =~ s{...}{$3-$1-$2}r;
[download]

Edit: Misread the order of comparison being used in the parent sample code.