I went with that recommendation:

-    like($@, qr/Cannot find directory\(ies\): $bad_gitdir/,
+    like($@, qr/\QCannot find directory(ies): $bad_gitdir\E/,
[download]

Although in a couple of places I had to use the \Q ... \E syntax twice within one pattern:

-    like($@, qr/Cannot find file\(s\) to be tested:.*$bad_target_args
+->[1]/,
+    like($@, qr/\QCannot find file(s) to be tested:\E.*\Q$bad_target_
+args->[1]\E/,
[download]

Thanks to all who responded. The revised code is available at Devel-Git-MultiBisect-0.07.tar.gz and should be available on CPAN within the hour. If anyone on Windows could give that a smoke test, that would be great.

Thank you very much.