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

##</code><code>##

a|b

##</code><code>##

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

##</code><code>##

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