I was thinking HERE docs are more likely. Put the bitshift inside a << section or stick it into a subroutine and have *that* do the bitshift.

#badrule : /reg1/i '=' sub_rule ',' /reg2/i '=' sub_rule ';'
#   {
#      $main::machineword = eval <<R
\$item[3] << 3 | \$item[7];
R
#      1
#   }
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Added: In conslutation on the chatterbox, some other ideas came up that won't step into string-eval which is really quite a hack. I'd use a unary '+' on the right-hand term. So instead of writing $item[3] << 3 it'd be $item << +3. The idea is to distinguish between a string and "other stuff." Another idea is to maybe put a newline between the end of the << and the next term and whatever other whitespace you prefer. Best of luck to you - report back, eh?