As others have suggested, BUILDARGS will do the job. Personally, I think you should consider:

1. my $date = MyDate->now;
2. my $date = MyDate->new(year => $y, month => $m, day => $d);
3. my $date = MyDate->parse("$y$m$d");
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There's no reason to force them all to happen through the same constructor method. #1 and #3 would just be wrappers for #2.