#! perl -slw
use strict;

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

print $re_a;
print $re_b;
print $re_c;
print $re_d;

__END__
(?-xism:\w+\#)
(?x-ism:\w+ \#
)
(?x-ism:\w+ [#]
)
(?x-ism:\w+ \#\\E\
)