Well uh... yes. Backreferences don't work, not as such, in a character class. the "\1" is interpreted as "\001", a chr(1). Try it: 
$_ = "a\001b";
 m[(.)([^\1])] and print "$1|$2";

[download]

|b
Oops.

There could be a way, but I'm not sure how, using the (?{...}) or (??{...}) constructs. I don't think I'd do it like that. Instead, I'd go for: 

$_ = 'aaab';
m[(.)(?!\1)(.)] and print "$1|$2";

[download]

a|b

In reply to Re: Regex backreference problem. by bart
in thread Regex backreference problem. by BrowserUk

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