Nonsense. The single quote you originally posted works just as well. 
my $s = q[<IL.Check/>];
my $r = q[<&#x221a;>];

print "Old word: $s\n";
$s =~ s/$s/$r/;
print "New word: $s\n";

[download]

There's no interpolation or escape sequences here. q vs qq is moot.

The real problem is that you are using text as a regex pattern without first converting it to a regex pattern. ("#" is special under "x".) Use quotemeta or its alias \Q..\E: 

my $s = q[<IL.Check/>];
my $r = q[<&#x221a;>];

print "Old word: $s\n";
$s =~ s/\Q$s\E/$r/;
print "New word: $s\n";

[download]

In reply to Re^3: How to ignore ampersand and pound signs? by ikegami
in thread How to ignore ampersand and pound signs? by Melk

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