Actually, Module::Build::Base uses ExtUtils::Manifest. Even worse, when you run "make dist" you get:

/usr/bin/perl -Iinc "-MExtUtils::Manifest=manicopy,maniread" \
        -e "manicopy(maniread(),'Mail-Mbox-MessageParser-1.21.0', 'bes
+t');"
[download]

This means my only real option is to copy ExtUtils/Manifest.pm to inc/ and then modify it. Bleh. Instead, I've asked the author to provide support for escaped spaces in the MANIFEST.

In the meantime, I've done the following:

1. comment out the lines in the MANIFEST that involve files with spaces
2. Put the following at the end of Makefile.PL:

     print "Fixing PREOP in the Makefile to copy files with spaces during
   + building\n";
   
     my $commands = q{; \$(CP) t/mailboxes/*\\\\ * \$(DISTVNAME)/t/mailbo
   +xes; \$(CP) t/results/*\\\\ * \$(DISTVNAME)/t/results};
   
     system "$^X -pi -e 's|^(PREOP\\s*=.*)|\$1$commands|' Makefile";
   [download]

The above code should work for both ExtUtils::MakeMaker and Module::Install.