It sounds like you want to compare two hashes, warning on any differences and updating so that values match the second hash? If I am right you might try something like this (pseudocode!):

## 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\tD
+ESIRED : $desired{$key}\n";
            ## updating
            $ENV{$key} = $desired{$key};
        }
    } else {
        warn ">$key< Only exists in DESIRED\n";
    }
}
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Also - multi dimensional hashes are coded as %hash = (key1 => {key2 => value},); or $hash->{key1}->{key2} = 'value'; see perlref. HTH.