I am happy and surprised to report that I found a solution to the problem: use the MacOSX 'arch' command.

[kw|~] arch -32 perl5.12 -ce 'use Mac::Glue'
-e syntax OK
[download]

Note that one needs the explicit "perl5.12" and not simply "perl", since (on MacOSX 10.8) /usr/bin/perl is a shell script which delegates to either /usr/bin/perl5.12 or /usr/bin/perl5.10.