Use the 'if' pragma. The condition must be on something which is available at compile time, unless you place it in a BEGIN block, like an environment variable, Config setting, or Perl built-in variable, for example:

use if ($ENV{IN_APP_SERVER}), 'MyApp::SOAPClient::SAP';
use if ($^O ne 'MSWin32'), 'POSIX';
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update: corrected Homerbug typo