This must be hooked up with the odd special case mentioned by davido that $. is hooked up with flipflops when thay have constant arguments. Dwimmerie gone berserk.

Not gone berserk, just an awkism. Convenient for processing on ranges of lines, e.g. (1..2)&&/foo/||(3..eof)&&/bar/. What sometimes surprises is that it applies to any constant, even nonnumeric. For instance:

perl -we'$. = "a"; "b"..die
Useless use of range (or flop) in void context at -e line 1.
Argument "b" isn't numeric in range (or flip) at -e line 1.
Argument "a" isn't numeric in range (or flip) at -e line 1.
Died at -e line 1.
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Here it happily compares int("a") to int("b"), giving warnings on both, and then (since both are 0, hence equal) dies. (Though older perls had a bug where it assumed not-equal if no reading of input had ever been done and $. was manually set.)