use strict;
use warnings;

my $teststring = qq/Hello# World/;

my $regex1 = qr{\QHello# World\E}x;
my $regex2 = qr{Hello\#\ World}x;
my $regex3 = qr{$teststring}x;
my $regex4 = qr{\Q$teststring\E}x;

print "String is $teststring\n";
print "Regex1 is $regex1\n";
print "Regex2 is $regex2\n";
print "Regex3 is $regex3\n";
print "Regex4 is $regex4\n";