On Windows, how can I check whether my Perl installation is 32 or 64 bit?
I'm reading the question to ask if Perl is compiled 64 bit, not if Windows or the CPU is.
Perl can be configured to use varying degrees of 64 bit-ness. You can look this up using the Config module.
To check if Perl is compiled to use 64 bit integers, you can look at the use64bitint entry in Config.
use Config;
print $Config{use64bitint};
define indicates yes.
There is also...
use64bitall meaning perl will be compiled to use all the 64 bit-ness it can, including 64 bit pointers allowing you to access more than 2 gigs of memory.ivsize indicating how many bytes Perl will use to store an integer, 8 indicates 64 bit.ptrsize is how many bits Perl will use to store pointers which allows you to use more than 2 gigs of memory per process, 8 indicates 64 bit.Common Config variables and their values can be seen in perl -V (note the capital V). Their definitions can be found with perldoc Config.
Note, you can compile Perl to use 64 bit integers regardless of whether your operating system or CPU is 32 or 64 bit. On a 32 bit CPU, Perl will use a type other than "integer" to store numbers, probably "long integer".