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

a|b

$_ = 'aaaabbbccddee';
while (m/(.)(.)(??{$1 eq $2})/g)
{
    print "$1|$2\n";
}

a|b
b|c
c|d
d|e