That is good to know that I can have both installed and use what I need by specifying it at run time. I run a few analysis programs, from time to time, that are huge with 100's of multi-megabyte data files and having a 64-bit system might speed things up.
I now have "arch = MSWin32-x64-multi-thread-5.14" and it is installed at "D:\Apps\Perl64". Having installed other Win32 applications, there already exists "D:\Apps (x86)", so that is where I will install the 32-bit version of Perl.
Given what you mentioned, there are apparently no environment variables or registry entries that will get confused with two versions on the same machine. Although, I wonder how PPM will distinguish between the two.
As for the loading of two libraries, even though I use a lot of disk space, I don't expect to run out of space anytime soon.
My sincere thanks to everyone that responded, I appreciate the help!