my %bitmask = (
refs => 0x00000002,
subs => 0x00000200,
vars => 0x00000400
);

sub bits {
    my $bits = 0;
    my @wrong;
    foreach my $s (@_) {
        push @wrong, $s unless exists $bitmask{$s};
        $bits |= $bitmask{$s} || 0;
    }
    if (@wrong) {
        require Carp;
        Carp::croak("Unknown 'strict' tag(s) '@wrong'");
    }
    $bits;
}