while ( <MYFILE> ) {
    chomp;
    my( $label, $index, $value ) = ( split /\s+/ )[3, 5, -1];
    
    $a_vals[ $index ] = $value;
    $b_vals[ $index ] = $value;
}

##</code><code>##

if ($label eq "A"){
$fields[-2] = $a_vals[ $index ]
    
   }
   if ($label eq "B"){
$fields[-2] = $b_vals[ $index ]

##</code><code>##

if ($label eq "A") {
    $fields[-2] = $a_vals[ $index ]
}
elsif ($label eq "B") {
    $fields[-2] = $b_vals[ $index ]
}
else {
    print "OOPS!  bad label\n";  # complain if something is fishy
}