I don't know a std Perl way to do this, but for example just a DIR C:\projects on my machine produces as the last line of output: 37 Dir(s) 275,221,610,496 bytes free.

On Windows, DIR on a volume will yield total bytes free on that volume. You may or may not be able to use all those bytes. You should also be aware that you should NEVER use more than 90%, preferably only 80% or NTFS will "auger into the ground".