1. Using CYGWIN

well, running perl Makefile.PL is not enough. this just builds the Makefile needed by your system to build the whole package using make.
so after saying perl Makefile.PL, try to do the following

make
make test
make install
[download]

in that order and see what happens.

2. Using ActivePerl

As Roger said above, you could use ActiveState's ppm (Perl Package Manager) which is imho a better choice on win32 systems (since we are not used to building stuff on windows :) )
doing just ppm install Data::Serializer is not always the best choice since there can be available multiple versions on multiple servers.
so open a dos-box (start/run/cmd) and then start ppm
use the command search to look up the version, so type ppm search Data::Serializer (or help search for assistance)

Have fun!