From looking at the module on CPAN, you have the right capitalization: it's use Rcs, not use RCS (although I don't see use RCS in the documentation -- maybe you have an old version?)
As to what you're trying to do: I haven't used RCS in a while, but as far as I know, there is no rcs get command. If you want the text of the file at the particular revision, it looks like you should be doing something like this:
my $rcs = Rcs->new;
# guessing the next line from your question
# equivalent to running "co -r 1.2" on the command line;
# that is, will retrieve version 1.2 without locking.
$rcs->co("-r" => "1.2");
At this point, the working file should contain the text
of the file at the specific version. Since you know its directory and its name, you can manipulate it like any other file.
Caveat: I haven't tested this code, and it's based on an old recollection of RCS. But I hope it will help point you in the right direction.