I always liked:

sub leap {
        use Time::Local;
        my ($mo,$yr)=@_; #assumes month in 0-11 form
        ($yr++, $mo=0) if ++$mo>11;
        return (localtime(timelocal(0,0,0,1,$mo,$yr)-86400))[3];
}
[download]

Seeing that someone else has done all the hard work for me.
Plus it has bonus inscrutableness.

--
$you = new YOU;
honk() if $you->love(perl)