Use the 'if' pragma. For example:

use if ($^O eq 'MSWin32'), 'Win32::Process';

use if ($^O ne 'MSWin32'), 'POSIX';
use if ($^O ne 'MSWin32'), 'POSIX' => ':sys_wait_h';

# Hack to allow compilation under UNIX 
# (NORMAL_PRIORITY_CLASS is Win32 only)
use if ($^O ne 'MSWin32'), 
      'constant' => 'NORMAL_PRIORITY_CLASS
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