If I understand your question.....

$param =~ s/'(\d+)'/$1/g;
$param =~ s/NULL/undef/g;
@list_of_params = split /\s*,\s*/, $param;
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Map would be useful if you were already dealing with a list. No point splitting things into a list before you've taken advantage regular expressions to substitute where necessary.

UPDATE: Thanks BrowserUK for the little tip. So as to actually substitute NULL with the undef value (rather than the 'undef' string), I propose this:

$param =~ s/'(\d+)'/$1/g;
@list_of_params = map { ($_ eq 'NULL') ? undef : $_ }
                  split( /\s*,\s*/, $param );
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Yes, it can be done in a one-liner using two map calls, but I still think that's making it a little too complex (and difficult to maintain in the future).

Dave

"If I had my life to do over again, I'd be a plumber." -- Albert Einstein