> Basically, you can't always parse a Perl program without running it.

That is true, but what the grandparent said is not: https://news.ycombinator.com/item?id=8626454

To expand on this, I think a succinct way of summarizing the issue is: you might have to run BEGIN blocks in Perl to parse the non-BEGIN-block part of the program. The canonical example of this is:

    BEGIN {
        if(arbitrary_function()) {
            eval "sub whatever() { }; 1" or die $@;
        } else {
            eval "sub whatever { }; 1" or die $@;
        }
    }

    # The parsing of this line depends on the result of arbitrary_function()
    whatever  / 25 ; # / ; die "this dies!";

arbitrary_function() can be anything, so the only way to parse the rest of the program is to actually run arbitrary_function().