A few points:

1. 'use' instead of 'require'. Then the inclusion is done at compile time and the subs can be found early by the compiler.
2. use warnings; That will tell you what's not to like. In older Perl, make that '-w' on the shebang line.
3. use strict; If you don't actually declare that, it can't tell you what's wrong.
4. You have globals in the library, and making them lexical does not improve matters. The place to deal with an instance variable like $user is in the main script, where it should be lexical. If you truly have global parameters, consider use constant ADMIN => 'admin';
5. At the top of your library, just say package Site::common; and save it as 'Site/common.pm'. No OO needed it can just be a bag of subroutines.

I recommend perlmod, perlsec, and perlvar.

Update: crazyinsomniac reminds me that warnings may also be turned on by $^W=1;.

After Compline,
Zaxo