This creates a copy (NOT an alias i.e. writing to it won't change the original data)

 %server_properties = %{$nodemetrics{server1}}

Otherwise

$h_server1 = $nodemetrics{server1}

(like already shown) is the same hashref, which can be safely changed

$h_server1->{cpu}=100 or alternatively $$h_server1{cpu}=100

see perlref and perldsc for details!