Without fully understanding your input data, simply setting $/ (which, in English, literally means INPUT_RECORD_SEPARATOR) to your separator (:?) could do exactly what you want.

my @data;
{
  local $/ = ":";   # or "\n:\n" for : on an empty line
  @data = <DATA>;
}
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This would put each "record" (set of lines) into individual elements of @data. It'd be up to you then to split on newlines or whatever, and to eliminate the trailing : on most records.