It's not sufficient to copy RNA.pm to some location where perl can find it.

You need to get the entire source distribution (of Makefile.PL, RNA.pm, RNA_wrap.c, and test.pl at least).
Then you need to build and test the module by running 'perl Makefile.PL' and 'make test'. In order for that to succeed, you'll need to have installed the viennaRNA library.
If all that works ok, you then run 'make install' to install the module.

Then you should be able to successfully 'use RNA;'.

Cheers,
Rob