You could store the rest of the line (each sub-entry) in a separate array, then loop over that. Here's one I put together quickly:

while(<INPUT>){
  my (@prefix,@rest);
  chomp;
  (@prefix[0..2],@rest) = split /,/;
  for (@rest)
  {
    local $, = ',';
    local $\ = "\n";
    print @prefix,$_;
  }
}
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