Select Code to Download

open my $F1, '<', $file1 
    or die "Cannot open file $file1 \n";
...
while (my $line = <$F1>) {
 ...
}

while (defined(my $f1 = <F1>) && defined(my $f2 = <F2>)) {
   ...
}

while ( 
        defined(my $line1 = <$F1>) 
...
{
   ...
}