## Hash 1 : %ENV
## Hash 2 : Desired
my %desired = ( A => 1, B => 2 ...);

## loop through Hash 2, checking against Hash 1
for my $key ( keys %desired){
    if (exists ($ENV{$key})){
        warn ">$key< exists in both hashes\n";
        if ( $desired{$key} eq $ENV{$key} ){
            ## simple text compare
            warn ">$key< has the same value in both hashes\n";
        } else {
            warn ">$key< has different values:\t ENV : $ENV{$key}\n\tDESIRED : $desired{$key}\n";
            ## updating
            $ENV{$key} = $desired{$key};
        }
    } else {
        warn ">$key< Only exists in DESIRED\n";
    }
}