Here's something a little different. I know you want to match on SOMETHING, but since you'll also need to match on DATE and SOMETHINGELSE (I'm assuming), why not match all three in sequence?

To me, this is a little simpler than the buffer ideas. It probably has some flaws, but it seems to be working.

my $tmp;
while ($tmp = <DATA>) {
   if ($tmp =~ /(DATE\d)/) {
      my %h = ();
      $h{'date'} = $1;
      $tmp = <DATA>;
      if ($tmp =~ /(SOMETHING\d)/) {
         $h{'match'} = $1;
         $tmp = <DATA>;
         if ($tmp =~ /(SOMETHINGELSE\d)/) {
            $h{'more'} = $1;
            print "$h{'match'},$h{'date'},$h{'more'}\n";
         }  else { redo; }
      } else { redo; }
   }
}

__DATA__
anything anything DATE1 anything
anything anything SOMETHING1
anything anything anything SOMETHINGELSE1

anything anything DATE2 anything
anything anything DATE3 anything
anything anything SOMETHING2
anything anything anything SOMETHINGELSE2

anything anything DATE4 anything
anything anything SOMETHING3
anything anything DATE5 anything
anything anything SOMETHING4
anything anything anything SOMETHINGELSE3
[download]

That returns:

SOMETHING1,DATE1,SOMETHINGELSE1
SOMETHING2,DATE3,SOMETHINGELSE2
SOMETHING4,DATE5,SOMETHINGELSE3
[download]

Hope this helps.