Is "changing the number" different from "getting the right answer"? If I play with putting other references to "@-" in the script, I get way different behaviors too -- e.g.:

#!/usr/bin/perl
use strict;
use warnings;

my $string = "abc";

while( $string =~ /b/g ) {
    print "@-\n";
}
[download]

produces:

*** malloc_zone_malloc[14337]: argument too large: 4294967280  Out of 
+memory!
[download]

Putting other kinds of references to @- before the loop still gives me a wrong answer.

In my case, it seems like the only time I get the right output is when the script uses "$&" (update: I also get the right output when I use parens in the regex). Are you able to get a right answer without this?

If you just get different wrong outputs, I'd say this would be expected from a bug that is probably causing some uninitialized memory location to be used the wrong way, and the value you get can vary depending on the instructions being compiled. What matters are the conditions that distinguish correct output from wrong output.