Use an inline thread:

use threads;

...

my $pid :shared;
my $tid = async{
    ## Removed 'my' below. Thanks Jettero.
    $pid = system("ssh root@ip ./sleepwake (args) &"); 
    $pid = 0;
    return $?;
};

...

## kill 2, $pid if $pid;

...

unless( $pid ) 
    my $result = $tid->join;
    ## do something;
}

...
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