splice @$fields, $i, 0;

##</code><code>##

>perl -wMstrict -le
"my $ar = [ qw(a b c d) ];
 ;;
 my $i = 0;
 for my $e (@$ar) {
   $e = uc $e;
   splice @$ar, $i, 0;
   ++$i;
   }
 ;;
 use Data::Dumper;
 print Dumper $ar;
"
$VAR1 = [
          'A',
          'B',
          'C',
          'D'
        ];