While I would not recommend it, $a and $b are only special in that they are predeclared and used by sort. Otherwise, you can use them like any other package variable:

sub mysort (&@)
{
    my $coderef = shift;
    while (@_ > 1)
    {
        $a = shift;
        $b = $_[0];
        my $c = $coderef->();
        print " $c";
        # do sorting
    }
    print "\n";
}

mysort { $a <=> $b } qw(1 3 2);
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But, it is a bad idea to use $a and $b except with sort

But, could use 2 other variables. Just have to declare them.

While I fully agree with Marshall that prototypes should generally be avoided (that is, unless you really know what your doing and why), and with you that, similarly, $a and $b should also be avoided most of the time, I think that we shouldn't be too dogmatic about that and that it does make sense to use both prototypes and the $a and $b special variables if you're going to write your own sort subroutine.

BTW, the List::Util module documentation illustrates some cases where it makes perfect sense to use the $a and $b variables in a context that has nothing to do with sorting.

In the example that I gave in Re: Prototype like sort()?, I'm localizing $a and $b before using them to avoid (at least, I hope) any side effects with their use somewhere else.