my $REPETITION_PAT = qr{

    \A         # Start at the beginning of the string
    (.+?)      # Match minimal initial sequence (as $1)
    \1*+       # Rematch it exactly as often as possible (maybe zero)

    # Then verify what's left is a proper truncation...
    (?(?{ index($^N, substr($_,pos())) }) (?!) )

}xms;