This does not exactly meet your requirements (no additional vars), but since everybody is going for crazy solutions and I have time to kill, here it is:

use v5.8;

$s = "abcdef"; # or initialize it yourself
($s, $_) = (undef, $s);

$s.=???!??:$&while/./g;

print "$s\n";
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Warning: Does not work correctly under Perls earlier than 5.8 (change in semantics). It can be fixed though, at the expense of aesthetics.