I'll admit to having very little idea of what problems that would cause for parsing. I guess the list could be bracketed, but that wouldn't look so nice.
Another alternative is to have the list terminated by the presence of the next keyword which may be possible, but could also be a problem depending on how the parser is implemented.
Another is that the absence of a comma after the final argument is indicative of the end of the parameters for that operator, so the next token must be a new operator (or the end of statement.
SQL parsers seem happy to cope with comma seperated lists of args between keywords. How much difference there is between that and parsing Perl I'm not sure.