When that happens to me, I typically don't bother looking for an eraser and just use my thumb and index finger to pick up the permanent marker in question.
I assume that this person did not need google to figure that out and was actually looking for ways to erase permanent marker ink from a flashlight lens or other hard non-pourus surfaces.
This also happens to me and when it does, I write over the permanent marker ink with a dry-erase marker and it all wipes off like magic. YMMV, especially if the permanent marker ink has been there a while.
I hope this helps the next person searching for "eraser gets permanent marker off flashlight lens."
I really don't mind writing an entry if it will help just one person, but I don't plan to address every interesting google search that comes my way.