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