# # if you know that there is no charge code 0 or
    # # empty string, you could compress that like so:
    # if ( my $code = $charge_codes{$src}{$dest} )
    # {
    #     ++$charges_against{$code};
    # }

    # # if you need multiple codes per src/dest pair:
    # my $charge_code_aref = $charge_codes{$src}{$dest}
    #   or next;
    # # increment the number of charges against each code
    # foreach my $code ( @$charge_code_aref )
    # {
    #     ++$charges_against{$code};
    # }