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if ($use_socket) {
  use Socket;         # oops!  Socket will get loaded at compile-time
                      # before $use_socket is even evaluated
}

if ($use_socket) {
  eval "use Socket";  # ok: Socket gets loaded at run-time
                      # if $use_socket is true
}

if ($use_socket) {
  require Socket;     # ok: Socket gets loaded at run-time
  Socket->import();   # if $use_socket is true
}

if ($use_socket) {
  require Socket;
  Socket->export_to_level(1, @_);
}