I think you need to close the connection in the thread that's running 'in the background'. Otherwise, the telnet client will wait for that duplicated connection to be closed, before it quits itself.

When I modify your code as follows, it works for me, i.e. telnet quits right away, as you expected:

sub sleeping
{
    close shift;
    ...
}

...
        elsif ( /^sleep|^s\n?\r?$/i )
        {
            threads->create( \&sleeping, $client )->detach();
        }
...
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