TMTOWTDI - Why search for things when regex will do that for you? It may not be the fastest, or it may, I don't care, but does that really matter?

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11107298
use warnings;

my $string ="test\nI want length of this line\n test";
my $position = 12;

pos($string) = $position;
$string =~ /.*\G.*\n?/;
my $length_of_line = length $&;

print "length of line at $position is $length_of_line\n";
[download]

Outputs:

length of line at 12 is 27
[download]