Well here is what it does appear to match on AS Perl 5.8 and on Perl 5.8 built from source on RH Linux(with a little help of use re 'debug'):

#! perl -slw
use strict;
#use re 'debug';

my $re_b = qr[\w+ #]x;
my $re_c = qr[\w+ [#] ]x;
my $re_d = qr[\w+ \Q#\E ]x;

print $re_b if  "foo" =~ $re_b;   #matches /\w/
print $re_c if  "foo#" =~ $re_c;   #matches /\w+#/
print $re_d if  "fldkdafds#\\E " =~ $re_d; #matches /\w+#\\E /;
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-enlil