What is the ruling on men using metal tools to curl eyelashes?

We do not know of any religious impediment to curling eyelashes, as long as it does not involve harm, or imitation of disbelievers or women. What is forbidden is plucking the eyelashes. Curling eyelashes does not alter Allah's creation; rather, it falls under the category of beautification. The general rule for things is permissibility unless there is evidence to prohibit them.