Sometimes (mainly when you're passing a file from one a process running under one user id to another process under a different user id) it makes sense to pass the filehandle and not the file. To do this, you'd want to use Socket::PassAccessRights as follows:

Socket::PassAccessRights::sendfd(fileno(SOCKET), fileno(FILE_DESCRIPTOR));

On the other end you'd want to receive like:

$file_descriptor = Socket::PassAccessRights::recvfd(fileno(SOCKET));

This unlikely to work on any system not purporting POSIX compliance. According to the author of the module, the socket passing code in Linux and FreeBSD is buggy, so use this module only when necessary.