I don't think the code does what you think it's doing:

If you copy'n'pasted the code, then the operators in bold tags, =-, won't do a regular expression match, and depending on what's stored in $_, you should get an Use of uninitialized value ... warning.

If you transferred the code by hand and made a typo just when typing it in, I think we'll need to get a more accurate error description of what goes wrong and how. I find it curious that your session IDs can only have two digits, and your action times have eight digits (HHMMSS), while your dates have 10 digits (YYYYMMDD) - something is very fishy in your format description.

Please take the time to post some relevant-yet-anonymized data and what you're actually trying to do with it. As your question title mentions splitting, maybe split is what you actually want to do:

my ($session_id, $last_action_date, $last_action_time) = split /-/, $s
+ession;
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Update: liverpole has even better eyes than I have - you need curly brackets for counting repetitions - what you're typing your code in from, it has a very bad font or you really need glasses.

Update2: Neither - nor : are what Perl considers a digit when matching against \d. The only chars allowable as digits are the following ten chars:

0 1 2 3 4 5 6 7 8 9
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I would still use split to get at the separate fields, or use unpack or a more specific regular expression:

# split:
my @fields = split /-/, $session;
die "Weird timestamp in >>$session<<"
    unless @fields == 5;
my ($session_id) = $fields[0];
my ($last_action_date) = join "-", @fields[1,2,3];
my ($last_action_time) = $fields[4];

# unpack:
my ($session_id,$last_action_date,$last_action_time) = unpack "A2xA10x
+A8", $session;

# regex:
$session =~ /^(\d{2})-(\d{4}-\d{2}-\d{2})-(\d\d:\d\d:\d\d)$/
    or die "Weird timestamp in >>$session<<"
my ($session_id,$last_action_date,$last_action_time) = ($1,$2,$3);
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You're not really doing a split here.  You're assigning variables to the results from a regular expression match.

Note a couple of things:

1. You should change =- to =~ (the regular expression binding operator (thanks jwkrahn, below!))
2. You shouldn't be capturing the values (10) and (8).  My guess is that you either meant "10" and "8" (without the parentheses), or (more likely) "{10}" and "{8}", to mean 10 digits and 8 digits respectively.

=~ is not the "regular expression operator", for example:

$string =~ tr/a-z/A-Z/;
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Notice that there is no regular expression involved in that example. Is the binding operator.

my ($session_id, $last_action_date, $last_action_time) = $session =~ /
+^(\d\d)-(\d{10})-(\d{8})$/;
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my ($session_id, last_action_date, $last_action_time) =
   $session =~
   m{^(\d\d)-(\d{4}(?:-\d\d){2})-(\d\d(?::\d\d){2})$};
[download]

Cheers

JohnGG