use strict;

package Bit;
use Tie::Scalar;

sub TIESCALAR {
    my ($self) = bless {}, shift;
    $self->{ref} = shift;
    $self->{mask} = shift;
    $self->{bit} = 0;
    return $self unless $self->{mask}; # mask can't be zero
    my $tmp = $self->{mask};
    while ( !($tmp & 1)) {
        $tmp >>= 1;
        $self->{bit} ++;
    }
    return $self;
}

sub FETCH {
    my $self = shift;
    return ( ${$self->{ref}} & $self->{mask} ) >> $self->{bit} ;
}

sub STORE {
    my $self = shift;
    my $value = shift;
    ${$self->{ref}} |= $self->{mask};
    ${$self->{ref}} ^= $self->{mask};
    ${$self->{ref}} |= $self->{mask} & ($value << $self->{bit});
}

#---------------------

my $bits = 0;     # a "n-bits" variable
tie my $a, 'Bit', \$bits, 0x07;  # bits 0..3 are $a
tie my $b, 'Bit', \$bits, 0x70;  # bits 4..7 are $b

$a = 1;
print "$bits\n";   # 1
$b = 1;
print "$bits\n";   # 17
$a = 0;
print "$bits\n";   # 16