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");
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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
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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 $1`;
print$\;}
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