Don't invoke "dpkg --print-architecture" for every call to
Dpkg::Arch::get_raw_build_arch().

diff -ru -x '*~' dpkg-1.16.9/scripts/Dpkg/Arch.pm dpkg-1.16.9-new/scripts/Dpkg/Arch.pm
--- dpkg-1.16.9/scripts/Dpkg/Arch.pm	2012-10-20 06:34:17.000000000 +0200
+++ dpkg-1.16.9-new/scripts/Dpkg/Arch.pm	2012-12-04 19:42:20.451002537 +0100
@@ -59,7 +59,7 @@
 	# dpkg-architecture itself, by avoiding computing the DEB_BUILD_
 	# variables when they are not requested.
 
-	my $build_arch = `dpkg --print-architecture`;
+	$build_arch = `dpkg --print-architecture`;
 	syserr("dpkg --print-architecture failed") if $? >> 8;
 
 	chomp $build_arch;