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
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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.