I added the line
Parse::RecDescent->Precompile($grammar, "Grammar");
and looked at the source of the parser that was created. @args is simply never set! It's simply inherited down from the parent production as if [ $item[2] ] wasn't there. So I tried to see if it worked with simpler expressions:

# NO WORKS:
funcdefine: /function/i name "(" parameter[ $item[2] ](s? /,/) ")" "{"
# WORKS:
funcdefine: /function/i name "(" parameter[ $item[2] ](s?)     ")" "{"
# WORKS:
funcdefine: /function/i name "(" parameter[ $item[2] ]         ")" "{"
[download]

So it's a bug in Parse::RecDescent in the (... /,/) code. btw, (... /,/) is implemented as a <leftop>, so you could try using <leftop> directly (might not work), or try using the code said to be equivalent to <leftop> in the docmentaton (will definitely work), or use my little hack below (will definitely work).

# HACKED TO WORK:
functiondefine: /function/i name "("
                {@arg=($item[2]);1}
                parameter(s? /,/)
                ")" "{"
[download]

I only renamed functiondefine above so that the lines wouldn't wrap.