How about a tied scalar operating as a boolean?

package Tie::MyBoolean;

sub TIESCALAR {
    my($class, $value ) = @_;
    my $boolval = ( $value ) ? 1 : 0;
    return bless \$boolval, $class;
}

sub STORE { 
    my $self = shift;
    ${$self} = ( shift ) ? 1 : 0;
    return ${$self};
}

sub FETCH {
    my $self = shift;
    return ${$self};
}

1;

package main;

use strict;
use warnings;

tie my $bool, 'Tie::MyBoolean';

$bool = 100;
print "$bool\n";
$bool = 0;
print "$bool\n";
$bool = ! $bool;
print "$bool\n";
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This has the characteristic of working a lot like $| (the autoflush special variable), which can only take on 0 or 1 as a value.