"(Hope I wrote that right ...)"

Instead of hoping, why not actually check your work. This took me just a few seconds:

$ perl -MO=Deparse -e '
sub sort_comp {
  my $result = ($a->{severity} <=> $b->{severity};
  if ($result == 0) {    # severity is the same -- dig deeper
    if ($a->{severity} eq 'critical'} {
      $result = ($a->{event_age} <=> $b->{event_age});   #asc
    } else {
      $result = ($b->{event_age} <=> $a->{event_age});   #desc
    }
  }
  return $result;
}
'
syntax error at -e line 3, near "};"
syntax error at -e line 5, near "critical}"
syntax error at -e line 7, near "} else"
Unmatched right curly bracket at -e line 12, at end of line
-e had compilation errors.
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After you've fixed your code, please ensure you follow the "How do I change/delete my post?" guidelines when correcting your what you wrote.

Hope I wrote that right ...

Update: Posted some almost working code: Re: merge two hashes into one. See also Reaped: Re^4: pseudocode for getting data from table.