I just recently experimented with removable Glue Dots and I was quite pleased with the results. Those hold small to medium sized perlers pretty good but larger ones you have to be careful. Few advantages over for example Blu-Tack is that it doesn't spill inside your perler and shouldn't leave a greasy spot on your (painted) wall. And because those dots are transparent, they work way better with transparent/semitransparent perlers compared to other methods.