Just stumbled across this.

Has it been resolved? That is, what is the definitive expected behavior/precedence wrt/ \Q\E and #?

Does the documentation need clarifying?

Update:

Just reran the example above on 5.10.1, still get the \E in the output:

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

produces:

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
----------------------------------------------------------------------
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And perlre for 5.10.1 says:

The # character is also treated as a metacharacter introducing a comment, just as in ordinary Perl code. This also means that if you want real whitespace or # characters in the pattern (outside a character class, where they are unaffected by /x), then you'll either have to escape them (using backslashes or \Q...\E ) or encode them using octal or hex escapes.

The 5.18 version is essentially the same.