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...