Both pfaut and cecil36 got part of the answer. The problem with pfaut's answer is that the symbol table (which, BTW, is in %main::, not %$main::) only lists non-my variables. Lexicals are in what's called a "scratchpad" which is much less easy to access from perl code. The problem with cecil36's answer is that perl is very difficult to parse, and since perl offers amazing introspection, you might as well use it instead of reimplementing perl.
The easiest answer is to use B::Xref, which will give you a nice cross-reference of where all variables are used, defined, etc. (That's actualy more information then you wanted. I assume it's fairly easy to change the output format, though.)
B::Xref, like the other B modules, looks through the internal bytecode generated by perl, so thus reuses perl's parser, meaning your gaurenteed to get the same interpratition as actualy running the code. (This doesn't mean that it's impossible to fool, just that it's more difficult. Using symbolic references will fool anything that doesn't profile the running code, as will eval STRING.
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