It looks like a problem with the module, because Windows detects and rejects attempts to bind to the same port twice.

To try it for yourself, run a copy of the below in one console and then try to run it again in a second session with the same port number:

#! perl -slw
use strict;
use IO::Socket;

my $lsn = IO::Socket::INET->new(
    Listen => 1, LocalPort => $ARGV[0]
) or die $^E;

while( my $client = $lsn->accept ) {
    close $client;
}
[download]

The second attempt will die with:

C:\test>lsn 1025
Only one usage of each socket address (protocol/network address/port) 
+is normally permitted at C:\test\lsn.pl line 5.
[download]