vec is useful for manipulating bit fields:

sub make_netmask {
    my $mask = shift;
    return if $mask < 0 or $mask > 128;
    my $str = "\0" x 16;
    vec( $str, 127-$_, 1) = 0b1 foreach 0..$mask-1;
    return map{ vec $str, $_, 8 } reverse 0 .. 15;
}
[download]