Of course you need to use the appropriate package variables. That said, I recind my suggestion in favour of the OPs use of our.

Not because of any problem with the use of qualified package vars, but because against my expectations, it runs far more quickly. Indeed, unless someone can show me the flaw in my benchmark, I'll be explicitly including our( $a, $b) in all my sorts from now on: 

#! perl -slw
use strict;
use Benchmark qw[ cmpthese ];

cmpthese -10, {
    "\$a<=>\$b"     => q[my @a = sort{                $b <=> $a   } 1 
+.. 1e6;],
    "\$::a<=>\$::b" => q[my @a = sort{              $::b <=> $::a } 1 
+.. 1e6;],
    "our(\$a,\$b)"  => q[my @a = sort{ our( $a, $b ); $b <=> $a   } 1 
+.. 1e6;],
};

__END__
C:\test>junk2.pl
            s/iter $::a<=>$::b     $a<=>$b  our($a,$b)
$::a<=>$::b   2.09          --         -0%        -77%
$a<=>$b       2.08          0%          --        -77%
our($a,$b)   0.469        344%        343%          --

[download]

UPDATE: I guess it really is time to upgrade. Under 5.10 I get much closer to what I expected: 

              Rate  our($a,$b)     $a<=>$b $::a<=>$::b
our($a,$b)  1.75/s          --        -46%        -46%
$a<=>$b     3.24/s         85%          --         -0%
$::a<=>$::b 3.24/s         86%          0%          --

[download]
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

In reply to Re^3: my $a outside sort block incompatibility by BrowserUk
in thread my $a outside sort block incompatibility 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.