foreach( keys %invalid ) {
        undef @tmp;
        @tmp = split $invalid{$_}, /\t/;
        while( @tmp ) {
            next if $_ =~ m/$fields[$_]/;
        }
    }

##</code><code>##

RECORD: while( <INPUT_FEED> ) {
    ....
    foreach my $invalid (keys %invalid ) {
        ...
        while( @tmp ) {
            next RECORD if ...;
        }
    }

##</code><code>##

   next if $_ = /$fields[$_]/;

##</code><code>##

    $PostCodeStrings{$fields[5]} =
        $PostCodeStrings{$fields[5]} .
        $fields[4] . "|" .
        $fields[5] . "|" .
        $fields[3] . "|" .
        $fields[2] . "|" .
        $fields[1] . "|" .
        $fields[0] . "\n";

##</code><code>##

    $PostCodeStrings{$fields[5]} .= 
        join( '|', @fields[4,5,3,2,1,0] ) . "\n";