I suggest Win32::Mutex as a way to guarantee only one instance of your Win32 program runs at a time. I have been using Win32::Semaphore but have learned that Inno Setup supports Mutexes (AppMutex). This allows my install to detect if my program is already running during install/uninstall and prompts the user to first shut down the app before continuing. For the benefit of others that may have the same need, I have included code for both.

Mutex Code

#!perl -w
use strict;
$|=1;
use Win32::Mutex;
my $mutexname="PIGMania";
if(Win32::Mutex->open($mutexname)){
    print "must be already running";
    exit;
    }
my $mutex=Win32::Mutex->new(1,$mutexname);
$mutex->wait(3);
print "Mutex: $mutex\n";
my $x=30;
while($x){
    print ".";
    select(undef,undef,undef,1.5);
    $x--;
    }
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Semaphore Code

#!perl -w
use strict;
$|=1;
use Win32::Semaphore;
my $semaphore_name="PIGMania";
if(Win32::Semaphore->open($semaphore_name)){
   print "$semaphore_name is already running\n";
   exit;
   }
my $semaphore=Win32::Semaphore->new(1,1,$semaphore_name);
print "Semaphore: $semaphore\n";
my $x=30;
while($x){
    print ".";
    select(undef,undef,undef,1.5);
    $x--;
    }
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