$seq =~ s{
  ([^U])  (?# anything other than U can be the "bracketing" character)
  (U+)    (?# match one or more Us)
  (?=\1)  (?# followed by the bracketing character again)
}{
  # replace the bracketing character and the Us that follow it
  # with an equal number of copies of the bracketing character
  $1 x (1 + length($2))
}xeg;  # replace all embedded sequences of Us in one go