splice @$fields, $i, 0;

[download]

Furthermore, and notwithstanding that a hash-based approach is, indeed, better for what you seem to want to do, the quoted splice statement from the OPed code also does not do what you want, removing zero elements (LENGTH == 0) beginning at an OFFSET of  $i in the array, i.e., it's an expensive no-op. 

>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'
        ];

[download]

(As a reinforcement to ikegami's point that you really don't want to add or delete elements in an array over which you are iterating, try changing the 0 in the  splice statement in the example code of this reply to a 1.)

In reply to Re: altering array in foreach during execution? by AnomalousMonk
in thread altering array in foreach during execution? by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.