That is the reason for a true value, 1

eval "use Cava::Pack; 1" or die "Oh noes! $@";
eval "use HOW::Cava::Pack; 1" or die "HOW!?! $@";
__END__
HOW!?! Can't locate HOW/Cava/Pack.pm in @INC (@INC contains: C:/perl/5
+.10.1/lib/MSWin32-x86-multi-thread C:/perl/5.10.1/lib C:/perl/site/5.
+10.1/lib/MSWin32-x86-multi-thread C:/perl/site/5.10.1/lib .) at (eval
+ 2) line 1.
BEGIN failed--compilation aborted at (eval 2) line 1.
[download]

Hmm, I don't recall installing Cava::Pack :)