You can use the Reuse argument in the IO::Socket::INET constructor:

my $lis = IO::Socket::INET->new(LocalPort => $portnumber,
                                Reuse     => 1,
                                Listen    => 10 );
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Doing this will set the SO_REUSEADDR option on the new socket. Without it, you'll have to wait a few minutes between exiting & restarting.