use List::Util 'max';
my @h_refs = ($hash_w, $hash_x, $hash_y, $hash_z);
my ($max, $max_i);
for ( 0 .. $#h_refs ) {
    my $max_v = max values %{ $h_refs[$_] };
    if ( ! defined $max || $max_v > $max ) {
        ($max, $max_i) = ($max_v, $_);
    }
}
print "The max value is $max\n";
print "It is in the hash referenced by index $max_i\n";