I think you are being bitten by the optimisation that translates {$a cmp $b} and {$b cmp $a} into direct sorts that do not use a callback. Effectively the block supplied, if one of a few pre-defined forms is used to compile in a different codepath at compile time.

About the best I've achieved in the past is to have both codepaths in the code and select between them yourself. 

$flag = 1; print $flag ? sort 'a'..'z' : sort {$b cmp $a } 'a'..'z';;
a b c d e f g h i j k l m n o p q r s t u v w x y z

$flag = 0; print $flag ? sort 'a'..'z' : sort {$b cmp $a } 'a'..'z';;
z y x w v u t s r q p o n m l k j i h g f e d c b a

[download]

It means some repetition but avoids a callback and so speeds up the sorting. I've wished several times that there were sortAplhaAscending, sortAlphaDescending, sortNumericAscending & sortNumericDescending keywords in the language. The codepaths already exist in the runtime, but you have to use somewhat obscure syntax to get at them.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re: Choosing the sort routine on the fly by BrowserUk
in thread Choosing the sort routine on the fly by grinder

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.