Note that perl is written in C

True.

I don't think that C makes any guarantees for the availability of a 64 bit integer type.

True. In fact, considering the age of C, I don't even think 32 bit integers are garanteed.

Many other language implementations "solve" this problem by ignoring platforms or compilers that don't do what they want, but perl's focus is usually stronger on portability.

... true, but Perl can be compiled to have 64-bit integers even if the underlaying hardware is 32 bits. It's actually the C compiler that determines whether you can have 64 bit integers or not. And many modern C compilers, including gcc, allow for this. (But it has worked the other way around as well. About a dozen years ago, I worked on a 64-bit SUN platform, but the then version of gcc couldn't deal with 64 bits).

Note also that even if your hardware is 32 bits, and you don't compile Perl to have 64 bit integers, your Perl integers still aren't limited to 32 bits. If an integer value exceeds 32 bits, Perl silently upgrades it to a float, and will give "integers" up to 64 bits. If the value exceeds about 53 bits, the "integers" aren't exact any more, they will have some losses in their least significant bits (which may mean that if you add 1 to such an integer, it doesn't change value).