I tend to use double quotes, almost exclusively, because if I do want to interpolate something into it later, I needn't worry about also fixing the quotes around the string as well. The only time I use single quotes is when I am dealing with a string that I know will never have any interpolation needed because it is holding some constant string that probably contains $'s, @'s, or \'s in it. That's really the only time I use single quotes. Oh, and hash keys that can't be barewords but will be constant.

At all other times I use double quotes, because I'll invariably end up needing to interpolate something.