I'm not sure what your background is, but a good reference on language grammars for interpreters and compilers is
Principles of Compiler Design (the dragon book). This gives a really good description of different types of langauge parsing and how they work. I can't find a good reference on the Perl language grammar (anyone?); but it's somewhere to start.
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echo S 1 [ Y V U | perl -ane 'print reverse map { $_ = chr(ord($_)-1) } @F;'
Warning: Any code posted by tuxz0r is untested, unless otherwise stated, and is used at your own risk.