If expat is installed, but in a non-standard directory, then use the
following options to Makefile.PL:

    EXPATLIBPATH=...  To set the directory in which to find libexpat

    EXPATINCPATH=...  To set the directory in which to find expat.h

For example:

    perl Makefile.PL EXPATLIBPATH=/home/me/lib EXPATINCPATH=/home/me/include

Note that if you build against a shareable library in a non-standard location
you may (on some platforms) also have to set your LD_LIBRARY_PATH environment
variable at run time for perl to find the library.