sub read_double_file {
    my ($fh1, $fh2)                 = @_;
    my ($line_file_1, $line_file_2) = (scalar <$fh1>, scalar <$fh2>);

   (defined $line_file_1 or defined $line_file_2) ? ($line_file_1 //= '', $line_file_2 //= '') : ();
}

while ( my ($line_from_1, $line_from_2) = read_double_file($fh1, $fh2) ) {
    ...
}