If you are opening a bunch of files, it is a good idea to include the bad filename into the error message, like this:

foreach my $dat1 (@data1){
  foreach my $dat2 (@data2){
    my $path = "file_$dat1\_$dat2.txt";
    open (FH, "<", $path) or die "Error opening '$path': '$!'";
    .....do something......
  }
}
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