Let's assume the text is in the ASCII character encoding and that there is at least one period among the first 400 characters in the text.... I demonstrated the defect to you in the complete, ready-to-run Perl script I posted.

Twaddle! This code doesn't have a period within the first 400 characters:

#!/usr/bin/perl

use strict;
use warnings;
use open qw( :encoding(UTF-8) :std );
use Modern::Perl;

my $string = ('X' x 400) . '.';
say length $string; # Prints 401
$string =~ s[^.{1,400}\.\K.*?$][];
say length $string; # Prints 401
[download]

The first 400 characters are all 'X's. Ergo, your code demonstrates exactly nothing!

And I can safely assume the fact that you have dropped your \X stuff like a hot brick means that you've finally realised that that is a dead end also.

So, I was right. Nothing more than PST.

Your code still has the same defect in it.

#!/usr/bin/perl -l

use strict;
use warnings;

my $string = '.' x 400_000_000;

print length $string; # Prints 400000000

$string =~ s[^.{1,400}\.\K.*?$][];

print length $string; # Prints 401
[download]

It's so easily fixed it boggles the mind you still haven't figured out how to fix it.

Hm. Curiouser and curiouser. I haven't made any attempt to 'fix' it, because you (nor anyone) had demonstrated a problem with it. And you still haven't:

$s = '.' x 500;;
$s =~ s[^.{1,400}\.\K.*?$][];;
print length $s;;
401
[download]

I guess your system must be broken?

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.