I guess the warning you receive is related to the fact that the @a array is empty inside the code block you pass to sort.

I've tried to check the contents of the @a array using Data::Dumper: 
#!/usr/local/bin/perl -w
use Data::Dumper;
@list = (4,9,3);
my @indicies = sort { print Dumper(\@a); $a[$a] <=> $a[$b] } 0..$#list
+;
print ("\n @indicies");

[download]
And the output I got is this 
$VAR1 = [];
Use of uninitialized value in numeric comparison (<=>) at foo.pl line 
+4.
Use of uninitialized value in numeric comparison (<=>) at foo.pl line 
+4.
$VAR1 = [];
Use of uninitialized value in numeric comparison (<=>) at foo.pl line 
+4.
Use of uninitialized value in numeric comparison (<=>) at foo.pl line 
+4.

 0 1 2

[download]
I guess you really wanted to use $list[$a] instead of, say, $a[$a]?

UPDATE: But, hey, why did marlyn use both @a and @list? *note: check the Q&A*

_____________________ 
$"=q;grep;;$,=q"grep";for(`find . -name ".saves*~"`){s;$/;;;/(.*-(\d+)
+-.*)$/;
$_=["ps -e -o pid | "," $2 | "," -v "," "];`@$_`?{print"+ $1"}:{print"
+- $1"}&&`rm $1`;
print$\;}

[download]

In reply to Re: sorting and indicies by vladb
in thread sorting and indicies 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.