It says: perl="C:\Perl64\bin\perl.exe" "%1" %*, which is exactly what I found in the registry in the data field at HKEY_CLASSES_ROOT\pl_auto_file\shell\open\command. However, .pl is associated with C:\Perl\bin\perl.exe, i.e. not perl64.

That's very confusing. So which version (32-bit or 64-bit) did you install? And if you've installed both, which did you install last?

I tried to modify the command to remove the 64, but even though I thought I was running as administrator, the modify instruction wouldn't work.

Hm. How did you try to change it?

What happened when you tried to change it? ie. did you get any error messages?

The alternative would be to aim all my perl instructions to perl64 instead of perl.

If you can't change it one way, why would you be able to change it the other?

Is there any advantage to using perl64 (v. 5.16.3)instead of perl (v. 5.12.4) on this 64 bit machine?

There are several advantages to using a 64-bit build of perl (regardless of the version).

1. 32-bit perls are limited to 2GB of memory. With 64-bit you can use all the memory available on your machine.
2. For many operations, the 64-bit version will run the identical code more quickly -- substantially so in some cases.

Of course, there are also downsides:

• The same program and data will occupy more memory -- up to double for some types of data.
• It can be harder or even impossible to get some XS-based modules to compile under 64-bit.