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$seq =~ s{
  ([^U])  (?# anything other than U can be the "bracketing" character)
  (U+)    (?# match one or more Us)
...
  # 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

$seq =~ s{^(U+)}{"I" x length($1)}eg;
$seq =~ s{(U+)$}{"O" x length($1)}eg;