Problem: Turn a list of filehandles which may or may not be followed by
an poll (system call) event specification into a list of triples

<file descriptor>, <event spec>, 0

which can be used as input to a pack call generating a struct pollfd
array which can be used as input to poll. If there's no event
specification for a filehandle, POLLIN is supposed to be used for it.

After various attempts which all had much too much duplicated code, I
came up with the following, statefully recursive solution:

sub build_pfds
{ return pack('('.PFD.')*', &expand_fh);
}

sub expand_fh
{ return unless @_;
return (fileno(shift), &expand_ev);
}

sub expand_ev
{ my $ev;

$ev = @_ && !defined(reftype($_[0])) ? shift : POLLIN;
return ($ev, 0, &expand_fh);
}

I'd be very much interested in opinions on that.

PFD is a constant defined as 'iss', the layout of a struct pollfd.

The caller is responsible for providing a well-formed argument list, ie,
glob references optionally interspersed with event specifications ---
I'm not in the "must defend against buggy code written by other
people" camp.