Are your perl and XML::Parser compatible? That is, if XML::Parser is compiled with 5.6.1 and perl was subsequently upgraded to 5.8, you'd have binary compatibility issues and you'll have to recompile XML::Parser from the source.
I jsut had this same problem. I just downloaded expat from unixpackages.com site which needs your valid id and password to login. Solaris package which you just have to install with pkgadd pkgname. This will add expat library which will resolve this problem.