If you want the refcount, that is simple.

$s = "you are ".int(rand(11))." on a score of 0 to 10";
print "refcount is ".Internals::SvREFCNT($s)."\n";
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result

refcount is 1
[download]

Is the refcount the correct way to figure out whats being a memory hog? maybe yes, if you trying to look for something that is overreferenced, in most of the time no. I suggest looking at Devel::LeakTrace (used it personally, it never found a SV leak, it was a malloc C leak). Next question is, are your memory problems from the perl script you are running, or from XS modules you are running? And do you have a memory leak (steady slow increase in ram over time), or are simply trying to reduce runtime ram bloat of the process?

You problems can be as simply as an array that is shifted/pushed onto and never poped/unshifted, or it can be a circular reference, or a unfixable (by you, only by p5p) interp specific bug.

You say you want to know what to undef, was your script written entirely with global and the time cost to convert it to a "use strict" script is too high for you?

Also look at Devel::Gladiator, I've never used it personally.