When "Show Perl Internal variables" is enabled, the special variable @_ (parameter list) should be also displayed. As of 0.6.28, it's not visible because it does not have an entry in the symbol table of package main.
Where is the "Show Perl Internal variables" setting?
"Show Perl Internal Variables" is in the popup menu of the Variables view (small arrow symbol) - same place as the Show Local Variables / Show Global Variables settings.
Start a debugger session and take a close look at the Variables view