Can I set any variables which would apply the -DPURIFY after Perl has already been compiled

Unfortunately not, it's a compile-time option only. You can read a bit more about this option and its primary use in perlhack, section External Tools for Debugging Perl.   Also, I suspect you'd incur a noticable decrease in performance with -DPURIFY (even when not running under Purify, valgrind, etc.); in other words, it's probably a matter of trading speed for memory control... (I haven't done any benchmarking, though.)

is there a variable which can be examined which I can use to monitor MEM usage

That would definitely be a nifty feature, but I don't think such a variable exists.   So, in addition to the respective system tools (like top on Unix — not sure what the best tool on Windows is), you might try the module GTop (based on libgtop from GNOME), but I don't know how well - if at all - that'd work on (native) Windows. Grepping through libgtop's configure file produces a couple of hits in connection with Cygwin and MinGW, but that's about all I can tell...