It's already explained why this happens, because the character class is created at compile time. But this gives us the right hint to "fix" the regex: use a delayed regex. 
($_ = 'aaab') =~ m[(.)((??{"[^$1]"}))] and print "$1|$2";
a|b

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Abigail

In reply to Re: Regex backreference problem. by Abigail-II
in thread Regex backreference problem. by BrowserUk

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