foreach my $dat1 (@data1){
  foreach my $dat2 (@data2){
   open (FH, "<", "file_$dat1\_$dat2.txt") or die "a horrible death $!";
   .....do something......
  }
}

##</code><code>##

open (FH, "<", "file_1.1_2.1.txt") or die "a horrible death $!";