$a and $b are not Perl predefined variables. So they do not belong into perlvar :-)
They are only of special use and meaning in a sort. So one might consider the occurance of $a and $b oustide of that as an error, but if that would raise an error you'd loose the ability to write custom sort subs.
Regarding your notes on the perl manpages, do you think that the unix manpages are better organized?
Well I think, if one is going to learn perl $a and $b should be noted along with the usage of strict, so one always remembers, that $a and $b will be used by a sort and so one better does not use them unless one explicitely wants to.
Having said that I think $a and $b are noted in the right place in the right document in a good (but worth improving) documentation.

Have a nice day
All decision is left to your taste

In reply to Re: Re: Re: (re:x5 use strict....)$a and $b should be in perlvar by little
in thread use strict won't require explicit name for all variables? by Biker

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.