Take another look at variable declaration. You $MM is local, but the otehrs are global. You want:

my ( $MM, $DD, $YY, $YYYY );
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If you don't assign anything to them, they wind up undef, which is the same value you explicitly assign to them.

The first two date formats differ only in zero-padding the day. Both will use the same split code, the only difference will be the precise return value. Similarly, the slash-delimited formats differ only in the segment of the hash used in looking up the corresponding month; otherwise they are identical.

Now if you want a REAL challenge, figure out how to differentiate between US, English and European formats:

• mm/dd/yy
• dd/mm/yy
• yy-mm-dd