and see if I still get $a and $b for free

You can get at them:

use warnings;
use strict;

{   package Bar;
    my $obj = Foo->new;
    print join ", ",
        sort {$obj->mysort($a,$b,"bar")} qw/x a o/;
}

{   package Foo;
    sub new { bless {}, shift }
    use Data::Dump 'pp';
    sub mysort {
        pp \@_;
        $_[0] cmp $_[1];
    }
}

__END__

[bless({}, "Foo"), "x", "a", "bar"]
[bless({}, "Foo"), "x", "o", "bar"]
[bless({}, "Foo"), "o", "a", "bar"]
x, o, a
[download]

Updated example: Previously I had mysort reaching into the main package via $::a, which is obviously not the best solution, now I'm passing $a and $b as parameters. (I see the AM made the same suggestion.) Update 2: There are other solutions possible as well.

Hope this helps,
-- Hauke D