select( (select (DM), ($- = 0))[0] );

##</code><code>##

for my $file (@files) {
   open my $fh, '<', $file;
   select((
      select($fh),
      $~ = "DM",
      $^ = "DM_TOP"
     )[0]
   );
   # Data adquisition stuff here
   write $fh;
}