Hence the speculation.

It was pointed out to me that, in some circumstances, that using lexicals rather than globals is quicker. One explaination I saw, but cannot now find, is that it is quicker to find a lexical than a global? I took this to be something to do with the fact that the compiler has to know where a lexical is at compile time, and effectively hard codes the 'location' into the optree, but that globals are 'found' at each time, at runtime. The technical detail my be wrong but this simplified imagery "works for me" ... until a better explanation is available.

How far this case extends I have never tried to assertain as I have found very few case where I use globals, and in the few cases I do, they are never citical to performance, but the the OP seemed to be looking for 'simple' measures that might help. Adding use strict at the top of a module will rapidly spit out the names of the globals. Adding one line, our ($a, $b, $this, $that); with an appropriate comment seem a simple enough change to at least make it worth trying, given the lack of other information and the restrictions imposed by the OP's question.