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my @fields = ( split /\s+/ );           # need to hold on to all field
+s for later
my( $index, $label ) = @fields[4, -1];  # this is an array slice

use 5.010;

...
    when ( 'B' ) { $fields[ -2 ] = $y; }
    default { say "OOPS!  bad label"; }
}

say join "\t", @fields;    # separates fields with TAB