It is bound to require more detail than this, but on the basis of the scant information in your post, this could be all you need:

use threads;

...
async{
    $RAMthread = threads->new(sub{$self->GetFreeRAM($serial)});
    $heapthread = threads->new(sub{$self->Getheap($serial)});
    $cputhread = threads->new(sub{$self->Getcpu($serial)});
    ## Presumably you do something with teh above agathered informatio
+n...
    ... # do it here
}->detach;

## get on with other stuff
...
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