I'm confused: the regexp you show should match an empty string. It looks like this:

  /
    ^                  # from the start
    (                  #   match
      ( [\w\d\-\_]+ )  #     one or more of [class]
      [:,]             #     followed by a delimiter
    |                  #   or match
      # nothing        #     nothing
    )                  # then
    ( .* )             #   match zero or more characters
    $                  # .. until we reach end of string
  /x
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This will match against any string that doesn't have newlines in it, and since undef evaluates to the empty string in a string context, it will match (taking advantage of the "or match ... nothing" option).

I don't have perl5.005_02 here, but I tried this:

  perl -wle 'print "ok" if undef =~ /^(([\w\d\-\_]+)[:,]|)(.*)$/'
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.. against each of 5.004, 5.004_05, 5.005_03 and 5.6.1 and it printed the expected warning and the expected "ok" in each case.

Can you come up with a short example like mine that prints "ok" on one of those perl builds, but not on the other?

Hugo