You can get the byte offset past each newline into an array like this:

our @ofs = (0);
push @ofs, pos while $string =~ /(\n)/g;
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Then you can find the line number and line offset of a given byte offset by calling:

sub ofs2line {
    my $offset = shift;
    for (0 .. $#ofs) {
        return ($_, $offset-$ofs[$_-1]) if $offset < $ofs[$_];
    }
    return ();
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That takes offsets to start with zero. Untested, beware fencepost errors.