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

a|b
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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]