$s1 = $s2 = "foo\(bar\)";

$b1 = "foo\(bar\)";
$s1 =~ s/\Q$b1\E/replace/;
print "$s1\n";

$b2 = qr/foo\(bar\)/;
$s2 =~ s/$b2/replace/;
print "$s2\n";