POSIX docs warn that it imports everything by default, so either give it an import list, or give it an empty import list and fully qualify the functions:

use POSIX 'strftime';
$now = strftime("%a, %e %b %Y %T GMT",gmtime($now));

use POSIX ();
$now = POSIX::strftime("%a, %e %b %Y %T GMT",gmtime($now));
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Time::Piece docs say to use its version of strftime and strptime "without the overhead of the full POSIX extension".