my $string ="test\nI want length of this line\n test";
my $position = 12; # Note this postion can be any position in any line. currently considering it inside 2nd line
my @newlines; # for storing \n positions 
push @newlines, 0;
while ($string =~/\n/g) {
    push @newlines, pos($string);
}
my itr = scalar @newlines - 1;
while ($newlines[$itr] > $position) {
    $itr--;
}
my $length_of_line = $newlines[$itr + 1] - $newlines[$itr];
#Better efficient solution to find length of line if we have only any position inside it. Thanks in advance!!!