Three problems here. First, $ a special meaning, so '$$' means something else than two $'s in a row. But second, $target does not actually end with $$, so you will not get a match. Third problem, you're executing grep in scalar context, so all you get is the number of matches, not the matches. Perhaps you want (untested):
my @matches = grep /\$\$$/, split ' ', 'PARJT## PARJT$$ PARJT##';