You're @INC consists of "C:/Program Files/Perl/perl/site/lib", "C:/Program Files/Perl/perl/lib" and "." (the current directory). Based on that, one would expect that, having installed Math::GSL, the file C:/Program Files/Perl/perl/site/lib/Math/GSL/Histogram.pm would now exist, but it obviously doesn't. It's absence is *not* a good sign.
Do you guys think I should reinstall perl?
Yes, I think so. Installing a module using PPM should not present these types of problems - and the fact that you're still getting errors with this PPM installation of Math::GSL suggests to me that something has got broken.
Also, it's not a good idea to install perl into a directory whose name contains any spaces (such as "Program Files"). Doing so can create difficulties. It doesn't matter much where perl is installed, but it's definitely best to avoid directories like "Program Files" - there's nothing wrong with installing into simply C:\perl.
So yes - I would perform a fresh install of perl. Then run the 'ppm install' command to install Math::GSL (and the 2 gsl DLL's). That's all you should need to do.