Generally, a 64 bit processor can help you with three things:

• Faster calculations with big numbers
• Accessing more than 4 GB of memory
• Using SIMD instructions on wider registers (128 bits for the PPC970)

(Okay, so this is a bit simplified, but that's the benefits for most people.) A 64 bit processor is not automatically twice as fast as a 32 bit processor.

IMHO, it is actually better for perl to stay 32 bit for a while: perl is known to be a memory hog (as a trade-off for higher speed), and going 64 bit would mean using double the amount of memory for pointers. Which is just not worth the marginal benefits.

My guess is that perl's 64 bit support will be enhanced for those that need it, and will stay an optional switch during compilation for quite some time to come. 64 bit processors like AMD's Opteron or IBM's PPC970 (hopefully to be used in Apples soon :)) can handle 32 bit code just fine.

Update: The PowerPC 970 is made by IBM, of course, not by Intel... thanks, elusion!