open (FH3, ">lessons.txt");
### You should *always* check the return from any calls to the system.
open FH3, ">lessons.txt" or die "lessons.txt: $!\n";
### You should also close this filehandle; don't assume that your file has actually
### been written before you do so!
close FH3;

$l="lessons.txt";
open(FH3, $l) || die("Could not open file!");
### Don't create variables that you're only going to use once; just use the value.
### *And* check the return of the call. And don't pre-assume the reason for failure:
### find out what it is with the '$!' (actual error) variable.
open FH3, "lessons.txt" or die "lessons.txt: $!\n";