Launch the application and find the id of the new window
Make sure there is exactly one such application running
Run the code twice without closing the window and see the error message.

Use the start command in order to detach the calc.exe from our Perl
script and both will be able to run simultaneously.
examples/Win32GUI/calc01.pl
# launch the application and find the id of the new window
# Make sure there is exactly one such application running
# Run the code twice without closing the window and see the error message 
# using the current public release from CPAN / installed by ppm
# There is a much newer version on the yahoo group web site that can be also used
# and has a lot of other functions

use strict;
use warnings;

use Win32::GuiTest qw(:ALL);

system "start calc.exe";
sleep(1);
my @windows = FindWindowLike(undef, "Calculator");
print join ":", @windows, "\n";

if (not @windows) {
    die "Could not find Calculator\n";
}
if (@windows > 1) {
    die "There might be more than one Calculators running\n";
}