## untested "idea" code ##
package TwoWayHash;
use overload '%{}' => \&as_hash;

sub new {
    my $self = shift;
    my $hashref = shift;
    my %struct = ( 'data' => $hashref );
    
    bless \%struct, $self;
}

sub as_hash {
    my $self = shift;

    # this will only work for simple hashes, really
    my $temp = reverse $self->{data};
    return $temp{shift};
}

##</code><code>##

use TwoWayHash;

my %a = get_results_from_query; # assume this populates %a;

# then, when you need $b{$a}:
{ 
    my $b = TwoWayHash->(\%a);
    print "I need ",$$b{'item'};  ## finds key where $a{key} eq 'item'
}

##</code><code>##

print "I need", rv_lookup('item',\%a);