With regards to getting the OS name, from perldoc perlvar:

       $^O     The name of the operating system under which this copy 
+of Perl was built, as determined during the configuration process.  T
+he value is identical to $Config{'osname'}.  See also
               Config and the -V command-line switch documented in per
+lrun.

               In Windows platforms, $^O is not very helpful: since it
+ is always "MSWin32", it doesn't tell the difference between 95/98/ME
+/NT/2000/XP/CE/.NET.  Use Win32::GetOSName() or
               Win32::GetOSVersion() (see Win32 and perlport) to disti
+nguish between the variants.
[download]

And with regards to your main question, I believe that a BEGIN block is probably what you need. From the Camel (3rd Edition, Chapter 18):

"Because a BEGIN block executes immediately, it can pull in subroutine declarations, definitions, and importations before the rest of the file is even compiled. These can alter how the compiler parses the rest of the current file, particularly if you import subroutine definitions."

Hope this helps,
Darren :)