Remove -foo ambiguity warning

$ ./perl -e 'sub foo{} -foo'
Ambiguous use of -foo resolved as -&foo() at -e line 1.

There is no ambiguity there, since unary minus does not force
its argument to be a bareword; it simply exempts barewords from
strictures.

This warning also makes it harder to use constantly recently added
to POSIX.  To avoid that warning, for negative infinity you have to
write ‘- Inf’ or ‘-+Inf’, because the most natural way of writing it,
‘-Inf’, warns.

See <20140901014232.20757.qmail@lists-nntp.develooper.com> and
related messages.