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 $length_of_line;
if ( $string =~ /\n.*(?=\n)/ && $-[ 0 ] < $position && $+[ 0 ] > $position ) {

    $length_of_line = $+[ 0 ] - $-[ 0 ];
    }