Usually you would handle this type of "single instance" issue with some sort of file locking mechanism. In doing a quick search on CPAN you might consider using Win32::Mutex or File::Flock.

I haven't used either of those as the company I work with has our own in-house module to handle mutex situations.

Here's a quick example (untested):

use File::Flock;

# the following croaks if it fails
my $lock = new File::Flock("C:\lock_file");

# do whatever here

# this will happen automatically once $lock 
# goes out of scope but it doesn't hurt to do 
# it explicitly once you're done with the lock
$lock->unlock();
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