I did the following:

use strict;
use warnings;
use YAPE::Regex::Explain;

my $regex1 = qr{\QHello# World\E}x;
my $parser1 = YAPE::Regex::Explain->new($regex1)->explain;
print "$parser1\n";

print "*" x 20;
print "\n";

my $regex2 = qr{Hello\#\ World}x;
my $parser2 = YAPE::Regex::Explain->new($regex2)->explain;
print "$parser2\n";
[download]

This returned:

The regular expression:
(?x-ims:Hello\#\ World\\E)
matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?x-ims:                 group, but do not capture (disregarding
                         whitespace and comments) (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n):
----------------------------------------------------------------------
  Hello                    'Hello'
----------------------------------------------------------------------
  \#                       '#'
----------------------------------------------------------------------
  \                        ' '
----------------------------------------------------------------------
  World                    'World'
----------------------------------------------------------------------
  \\                       '\'
----------------------------------------------------------------------
  E                        'E'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
********************
The regular expression:
(?x-ims:Hello\#\ World)
matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?x-ims:                 group, but do not capture (disregarding
                         whitespace and comments) (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n):
----------------------------------------------------------------------
  Hello                    'Hello'
----------------------------------------------------------------------
  \#                       '#'
----------------------------------------------------------------------
  \                        ' '
----------------------------------------------------------------------
  World                    'World'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

Note the addition of the "\E" at the end of the first explanation, but not the second.

Note also that the "\Q" is dropped by YAPE::Regex::Explain.

The behaviour is constant across 5.8.9, 5.10.1 and 5.12.1 with YAPE::Regex::Explain 3.011

HTH

UPDATE:
From perldoc perlre:

(?#text)
A comment. The text is ignored. If the /x modifier enables whitespace formatting, a simple # will suffice. Note that Perl closes the comment as soon as it sees a ), so there is no way to put a literal ) in the comment.

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