What you would want to do, then, is use each one in a loop, rather than getting them ahead of time. That is, rather than:

my @drives = (
  Win32::GetNextAvailDrive(),
  Win32::GetNextAvailDrive(),
  Win32::GetNextAvailDrive(),
  Win32::GetNextAvailDrive(),
);
do_stuff_with(@drives);
[download]

do this:

for (1..4)
{
  my $drive = Win32::GetNextAvailDrive();
  do_stuff_with($drive);
}
[download]

Hopefully, do_stuff_with() will net use or something so that the drive becomes unavailable (in use) so the next time through, Win32::GetNextAvailDrive will get a new drive.