'2 apples' + '2 oranges' == 4

I can't find the documentation for the behavior illustrated in the
Subject line (i.e. in Perl arithmetic operations are defined for
strings beginning with numbers[*]). Any pointers would be appreciated.
(Also, meta-pointers on how one would go about finding this
documentation if one didn't know already would also be very welcome.)



* Actually, the format of the strings Perl will perform arithmetic
on is more compilicated than I described above. Optional leading
whitespace is permitted, for example.

To s&e^n]d me m~a}i]l r%e*m?o\v[e bit from my a|d)d:r{e:s]s.