Unfortunately, this isn't thorough. Leap years don't occur if the year is a centennial year (although they do occur every 4th century). Here is what we in some of the NMS scripts:

sub is_leap {
  my $y = shift;
  return (!($y % 100) && !($y % 400)) || (($y % 100) && !($y % 4));
}
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It's probably easier to just write out what you mean:

sub is_leap {
  my $y = shift;
  return 0 if $y % 4; # not multiple of 4
  return 1 unless $y % 400; # is multiple of 400
  return 0 unless $y % 100; # is multiple of 100
  return 1; # everything else
}
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-- Randal L. Schwartz, Perl hacker