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

[download]
Prints 
a|b

[download]
Another cool way to do the same match is with the match-time pattern interpolation technique:
$_ = 'aaaabbbccddee';
while (m/(.)(.)(??{$1 eq $2})/g)
{
    print "$1|$2\n";
}

[download]
Prints...
a|b
b|c
c|d
d|e

[download]

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

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