I do something similar in scripts that automate module installation. However, I use a longer list of variables:

$makefile_args =
  " INSTALLPRIVLIB=$prefix/usr/lib/perl5/5.6.0 " .
  " INSTALLARCHLIB=$prefix/usr/lib/perl5/5.6.0/i386-linux " .
  " INSTALLSITELIB=$prefix/usr/lib/perl5/site_perl/5.6.0 " .
  " INSTALLSITEARCH=$prefix/usr/lib/perl5/site_perl/5.6.0/i386-linux " .
  " INSTALLBIN=$prefix/usr/bin " .
  " INSTALLSCRIPT=$prefix/usr/bin " .
  " INSTALLMAN1DIR=$prefix/usr/share/man/man1 " .
  " INSTALLMAN3DIR=$prefix/usr/share/man/man3 ";

All I do is set the $prefix. Voila! Modules where we want 'em!