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system ('ssh -O exit localhost 2> /dev/null;');
use Expect;
...
$su->soft_close(); # this step hang 15 seconds
# $su->expect(undef);
exit 0;

system ('ssh -O exit localhost 2> /dev/null;
    ssh -M localhost -fN 2> /dev/null');
...
$su->soft_close(); # this step now will not take more than 1 second
#$su->expect(undef);
exit 0;