It seems that with Net::DBus you need to call get_service and then call get_object on the service object.

I am not running gnome so I can't test this, but I think that this code will get you closer:

use Net::DBus;
#get the session bus
my $bus = Net::DBus->session;
my $srv = $bus->get_service("org.gnome");
my $devobj = $srv->get_object("/org/gnome/PowerManager", "org.gnome.Po
+werManager");

my $cookie = $devobj->Inhibit("Squeezecenter", "Playing");
#.... more code
$devobj->UnInhibit($cookie);
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