while (<>) {
    chomp;
    my @values = (m/
        (.)              # One character
        (?!\1)           # not followed by that character
        (?=.)            # but followed by a character
    /gx);
    print join(",", @values), "\n";
}
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This uses zero length lookaheads to test first that the same character does not follow, and then also that at least one character follows.

Update: Removed needless $_ =~.