In the updated example, the string '00000000000000000000' would not match the regex you specify for session (which expects hyphens), and therefore the contents of $session_id, $last_action_date, and $last_action_time would all be undefined. I usually do something like this (admittedly more verbose code):

my ($session_id, $last_action_date, $last_action_time) = (0,0,0);
if ($session =~ /^(\d\d)-(\d{10})-(\d{8})$/) {
  $session_id = $1;
  $last_action_date = $2;
  $last_action_time = $3;
}
else {
  # print some kind of warning -- the session ID didn't match my regex
+!
}
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I have generally found that reliance on a regex match for code flow can be problematical ... external data is not always what I think it will be. You need to protect yourself from disaster a little more effectively.

Update: I see that you updated your session example ... even so, the rest of the post has some merit, I think. I would probably use split /-/ for that particular case, anyway, and test that the split produced the requisite number of tokens before I rushed off to use the data.