Sepia aims to make Emacs the kind of interactive development environment for Perl that it already is for Emacs Lisp. This involves a number of components:
die, and Devel::LexAlias to inspect and change lexicals.
Not having used Devel::PerlySense, I won't try to make a detailed comparison, but my impression is that Devel::PerlySense is geared toward off-line development (uses PPI, has a class browser, etc.), while Sepia is geared toward on-line development (supports interaction, value inspection, debugging). So give both a try, and see which style suits you best.