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I think that this is probably the most 'legitimate' use of a source filter I've seen. I really like that it turns the usual practice on it's head and only imposes additional overhead (minimal, compile-time) when enabled, whilst imposing none when disabled. Nice++
However, I am also a little dubious about the use of =begin as the trigger, and as the name of the module.
Whilst =begin DEBUGGING and =begin VERBOSE read quite nicely, and combining that with the requirement that the second word be all-uppercase makes it less likely that it will step on anyones toes, using what is a fairly common place word 'begin' as the trigger seems to invite the possibility that it will encounter a few modules containing a
It's a stretch, but using a less common, combined or made-up word might lessen the chances. The only (pathetic, cutesy) possibiliy that comes to mind is
Aweful, but it demonstrates the idea that a made up word that lends itself to immediate association with debugging, is less likely to turn up by accident, and less likely to be overlooked by the unfamiliar.
I also think that as a module name, 'begin' is most unlikely to attract my attention, or turn up on my keyword searches of CPAN for a debugguging aid.
I really do like the implementation though -- and I will be making use of it.