my @days = (
    [ qw(31 28 31 30 31 30 31 31 30 31 30 31) ], # non-leap
    [ qw(31 29 31 30 31 30 31 31 30 31 30 31) ]  # leap
)

##</code><code>##

sub leap {
    my $y = shift;
    return ($y % 4 == 0 && $y % 100 != 0) || $y % 400 == 0;
}

##</code><code>##

my $leap = leap($y);

...

$days[$leap][$m]