tie my $hash_ref,
    'Tie::Constrained', sub { ref $_[0] eq 'HASH' };

tie my $scalar_ref,
    'Tie::Constrained', sub { ref $_[0] eq 'SCALAR' };

tie my $scalar_value,
    'Tie::Constrained', sub { not ref $_[0] };

tie my $query,
    'Tie::Constrained',
    sub { ref $_[0] and $_[0]->isa('CGI') },
    CGI->new;