And the source code for Date::Leapyear reads:

sub isleap {
    my ($year) = @_;
    return 1 if (( $year % 400 ) == 0 ); # 400's are leap
    return 0 if (( $year % 100 ) == 0 ); # Other centuries are not
    return 1 if (( $year % 4 ) == 0 ); # All other 4's are leap
    return 0; # Everything else is not
}
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which is already a provided answer. The same semi-correct formula is also used in Astro::Time and DateTime. Unless someone goes to the touble of encoding the entire FAQ, stick with the original answer. I like CPAN as much as the next Monk, but some modules are wasteful.

Updated: I do note that the DateTime::Calendar::* modules at least make some attempt at dealing with cultural and temporal variances ...