It is a POE::Preprocessor directive.

It's POE's way to use bytes; if the bytes pragma exists, or to fail gracefully if it does not exist (pre 5.6, IIRC).

Update: You find

use POE::Preprocessor ( isa => "POE::Macro::UseBytes" );
[download]

early on in the source of POE::Driver::SysRW, but for some reason POE::Macro::UseBytes is not indexed by CPAN. Download the source of POE if you want to see UseBytes.pm