#!/your/perl/here

use strict;
use warnings;

my $string = <<EOS;
one
two
three
four
five
six
EOS

my @strings = split "\n", $string;
my @lookup;
$lookup[0] = 0;
foreach my $i ( 1..$#strings )
{
    # byte offset of 1st char on this line
    $lookup[$i] = length($strings[$i-1]) + $lookup[$i-1];
}

my ($line, $offset) = find_line_offset( $ARGV[0] );
print "line: $line, offset: $offset\n";

exit;

sub find_line_offset
{
    my $offset = shift;
    
    my $lo = 0;
    my $hi = $#lookup;
    my $line;
    while ($lo+1 < $hi)
    {
        $line = int(($hi+$lo)/2);
        if ( $lookup[$line] > $offset )
        {
            $hi = $line;
        }
        elsif ( $lookup[$line] < $offset )
        {
            $lo = $line;
        }
        else
        {
            # small chance it's equal
            last;
        }
    }
    
    # could have overshot 1
    if ( $lookup[$line] > $offset )
    {
        $line--;
    }
    
    if ( $lookup[$line+1] > $offset )
    {
        # this is the one
        my $line_offset = ($offset - $lookup[$line]);
        return ($line, $line_offset);
    }
    else
    {
        return;
    }
}
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