I started with the most obvious one. In double quotes, you can use the "babycart" operator (see perlsecret); inside it, you can have any code you like (well, in our case, any code not containing a backslash, because, as haukex correctly found in perlop, you can't include a backslash literally in a qq\\). To most straightforward way is to use chr:

1.

qq\@{[chr 92]}\
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There are infinitely many ways how to get a backslash. We can use pack

2.

qq\@{[pack "c", 92]}\
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or sprintf

3.

qq\@{[sprintf '%c', 92]}\
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Another way is to use a bitwise operation, e.g. xor. Here are just two examples I liked:

4.

qq\@{['{'^"'"]}\
qq\@{['~'^'"']}\
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Then I skimmed perlfunc searching for functions that can return a backslash. I found a few that return a bit more so we need to combine them with substr or a match:

5.

qq\@{[substr quotemeta ";", 0, 1]}\
qq\@{[prototype("CORE::push") =~ /^(.)/]}\
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We can also require a core module that outputs a backslash.

6.

qq\@{[require File::Spec::Win32 and File::Spec::Win32->rootdir]}\
qq\@{[do{require Data::Dumper;substr Data::Dumper::Dumper("'"),-5,1}]}
+\
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OK. Is there any other way than using the array dereference? I've found a way! Let's use a string dereference instead. We need to create a string reference inside, which I can't do without a backslash, but fortunately there's eval that makes it possible to generate the backslash without using a backslash.

7.

qq\${eval chr(92).'"'.(chr(92)x2).'"'}\
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Finally, here's a joke. What a pity it doesn't work. Update: In a way, it does! See Tux's reply below!

∞.

qq\@{[reverse "/"]}\
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To sum it up:

print
qq\@{[chr 92]}\,
qq\@{[pack "c", 92]}\,
qq\@{[sprintf '%c', 92]}\,
qq\@{['{'^"'"]}\, qq\@{['~'^'"']}\,
qq\@{[substr quotemeta ";", 0, 1]}\,
qq\@{[prototype("CORE::push") =~ /^(.)/]}\,
qq\@{[require File::Spec::Win32 and File::Spec::Win32->rootdir]}\,
qq\@{[do{require Data::Dumper;substr Data::Dumper::Dumper("'"),-5,1}]}
+\,
qq\${eval chr(92).'"'.(chr(92)x2).'"'}\,
qq\@{[reverse "/"]}\;
[download]