The only specific enough mention I've found so far is section 2.6 of the Camel book, 3rd edition, where it says

Perl converts between the various subtypes as needed, so you can treat a number as a string or a string as a number, and Perl will do the Right Thing. To convert from string to number, Perl internally uses something like the C library's atof(3) function.

Of course, that assumes that you are familiar with the atof function, which is described in the manpage as

double atof(const char *nptr);

The atof() function converts the initial portion of the string pointed to by nptr to double.

I have to recognize that the PHP documentation is more explicit in this regard; see for example http://www.php.net/manual/en/language.types.string.php#language.types.string.conversion