provide default values for a hash

################
#COMPARE_N_SUBSTITUTE
#Compare a hash to a fall-through hash.  If any values are present in the fall-through and not in the current values then insert them in the DYC hash.
#compare_n_substitute(hash1, hash2)  #insert key-value-pairs from hash2 into hash1 only if those pairs are not defined in hash1.
#compare_n_substitute(\%hash, \%default)
sub compare_n_substitute(\%\%) {
    my ($hashref, $fallthrough_hashref) = @_;
    for my $key (keys %$fallthrough_hashref) {
	unless (defined($$hashref{$key})) {
	    $$hashref{$key} = $$fallthrough_hashref{$key};
	}
    }
}