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    foreach( keys %invalid ) {
        undef @tmp;
...
            next if $_ =~ m/$fields[$_]/;
        }
    }

RECORD: while( <INPUT_FEED> ) {
    ....
...
            next RECORD if ...;
        }
    }

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

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

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