#!/usr/bin/perl -w
use strict;

while (<DATA>)
{
   next if (/^\s*$/);    #skip blank lines
   chomp;
   
   my ($date, $backupset , $parm , $value) = parseline($_);
   
   # the idea is to concentrate the parsing of the line and its
   # associated "regex-foo" into one place. I think rest of your
   # code can use simple eq or ne comparisons.
   
   print "$date\n";
   print "   BACKUP SET = $backupset\n";
   if ($value eq "") 
      { print "   SINGLE TOKEN: $parm\n";}
   else
      {print "   PAIR:  $parm IS $value\n";}

}

sub parseline
{
   my $line = shift;
   my ($date, $rest) = $line =~ m/(^.*\d{4}):(.*)/;
   my ($backupset,  $msg) = split(/backup:INFO:/, $rest);
   $backupset =~ s/:\s*$//;  #trimming some unwanted thing like ':' is ok
   $backupset =~ s/^\s*backup\.//; #more than one step is just fine too!
   my ($parm, $value) = $msg =~ m/(.*)=(.*)/;
   $parm  ||= $msg;   #if match doesn't happen these will be undef
   $value ||="";      #so this trick makes sure that they are defined.

   return ($date, $backupset, $parm, $value);
}   

=prints...
Sun Aug 15 20:00:03 2010
   BACKUP SET = set2_lvm
   SINGLE TOKEN:  START OF BACKUP
Sun Aug 15 20:00:04 2010
   BACKUP SET = set2_lvm
   PAIR:   backup-set IS backup.set2_lvm
Sun Aug 15 20:00:04 2010
   BACKUP SET = set2_lvm
   PAIR:   backup-date IS 20100815200003
Sun Aug 15 20:00:04 2010
   BACKUP SET = set2_lvm
   PAIR:   backup-type IS regular
Sun Aug 15 20:00:04 2010
   BACKUP SET = set2_lvm
   PAIR:   backup-date-epoch IS 1281927603
Sun Aug 15 20:00:04 2010
   BACKUP SET = set2_lvm
   PAIR:   backup-directory IS /home/backups/backup.set2_lvm/20100815200003
Mon Aug 16 00:00:04 2010
   BACKUP SET = set1_lvm
   SINGLE TOKEN:  START OF BACKUP
Mon Aug 16 00:00:05 2010
   BACKUP SET = set1_lvm
   PAIR:   backup-set IS backup.set1_lvm
Mon Aug 16 00:00:05 2010
   BACKUP SET = set1_lvm
   PAIR:   backup-date IS 20100816000003
Mon Aug 16 00:00:05 2010
   BACKUP SET = set1_lvm
   PAIR:   backup-type IS regular
Mon Aug 16 00:00:05 2010
   BACKUP SET = set1_lvm
   PAIR:   backup-date-epoch IS 1281942003
Mon Aug 16 00:33:15 2010
   BACKUP SET = set2_lvm_lvm
   PAIR:   last-backup IS /home/backups/backup.set2_lvm_lvm/20100814200003
.... and so forth ....   
=cut

##</code><code>##

__DATA__
Sun Aug 15 20:00:03 2010: backup.set2_lvm:backup:INFO: START OF BACKUP
Sun Aug 15 20:00:04 2010: backup.set2_lvm:backup:INFO: backup-set=backup.set2_lvm
Sun Aug 15 20:00:04 2010: backup.set2_lvm:backup:INFO: backup-date=20100815200003
Sun Aug 15 20:00:04 2010: backup.set2_lvm:backup:INFO: backup-type=regular
Sun Aug 15 20:00:04 2010: backup.set2_lvm:backup:INFO: backup-date-epoch=1281927603
Sun Aug 15 20:00:04 2010: backup.set2_lvm:backup:INFO: backup-directory=/home/backups/backup.set2_lvm/20100815200003
Mon Aug 16 00:00:04 2010: backup.set1_lvm:backup:INFO: START OF BACKUP
Mon Aug 16 00:00:05 2010: backup.set1_lvm:backup:INFO: backup-set=backup.set1_lvm
Mon Aug 16 00:00:05 2010: backup.set1_lvm:backup:INFO: backup-date=20100816000003
Mon Aug 16 00:00:05 2010: backup.set1_lvm:backup:INFO: backup-type=regular
Mon Aug 16 00:00:05 2010: backup.set1_lvm:backup:INFO: backup-date-epoch=1281942003
Mon Aug 16 00:33:15 2010: backup.set2_lvm_lvm:backup:INFO: last-backup=/home/backups/backup.set2_lvm_lvm/20100814200003
Mon Aug 16 00:33:15 2010: backup.set2_lvm_lvm:backup:INFO: backup-size=424.53 GB
Mon Aug 16 00:33:15 2010: backup.set2_lvm_lvm:backup:INFO: backup-time=04:33:12
Mon Aug 16 00:33:15 2010: backup.set2_lvm_lvm:backup:INFO: backup-status=Backup succeeded
Mon Aug 16 00:33:15 2010: backup.set2_lvm_lvm:backup:INFO: Backup succeeded
Mon Aug 16 00:33:16 2010: backup.set2_lvm_lvm:backup:INFO: END OF BACKUP
Mon Aug 16 01:59:07 2010: backup.set1_lvm:backup:INFO: last-backup=/home/backups/backup.set1_lvm/20100815000006
Mon Aug 16 01:59:07 2010: backup.set1_lvm:backup:INFO: backup-size=187.24 GB
Mon Aug 16 01:59:07 2010: backup.set1_lvm:backup:INFO: backup-time=01:59:04
Mon Aug 16 01:59:07 2010: backup.set1_lvm:backup:INFO: backup-status=Backup succeeded
Mon Aug 16 01:59:07 2010: backup.set1_lvm:backup:INFO: Backup succeeded
Mon Aug 16 01:59:09 2010: backup.set1_lvm:backup:INFO: END OF BACKUP