Thank you all for your time and helpful advice (although I am baffled by Plankton's response! what does it mean?!?)
it turned out, in the end, that my problem was actually that one of the variables that i was trying to print out simply didn't exist - it never was one of the params that got passed to the script from the form i'm using. so that was simple. (there is no param called "SUBMITINST").

oddly enough, though, when i ran Anonymous Monk's snippet to spit out all the $vars, it ignored that?

i'm left with the question: "why does the compiler recognize that $submitinst as undef in line 33, but not in line 23? i.e. why didn't it complain that $in{SUBMITINST} was undef?

and thanks also to Ovid (whose tutorials i love!!) for pointing out that i can just use the hash values, rather than transferring them into new scalars. that cleans things up a lot!