open my $F1, '<', $file1 
    or die "Cannot open file $file1 \n";

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

##</code><code>##

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


##</code><code>##

while ( 
        defined(my $line1 = <$F1>) 
        and defined(my $line2 = <$F2>) 
      ) 
{
   ...
}