my $items = $farmdetails->{FarmRoleSet}{Item};
for my $item (@$items) {
	next unless $item->{RoleID} == $some_number;
	my $ExternalIP = $item->{ServerSet}{Item}{ExternalIP};
	my $InstanceID = $item->{ServerSet}{Item}{PlatformProperties}{InstanceID};
}

##</code><code>##

print Dumper($farmdetails->{FarmRoleSet}{Item}[0]);
print Dumper($farmdetails->{FarmRoleSet}{Item}[0]{ServerSet});

# or better:

my $items = $farmdetails->{FarmRoleSet}{Item};
for my $item (@$items) {
	print Dumper($item);
	print Dumper($item->{ServerSet});
}