my @cache;
my $found=0;
while (<$file>) {

  if ( ... found the match ...) {
    $found++;
  }

  if (/^<DataStart>/) {
    @cache=();
  }

  push(@cache, $_);

  if (/^<DataEnd>/ and $found) {
    Do_stuff_with_match(@cache);
    $found=0;
  }
}