# the way you want
if( magical_how_many_open() < $what_i_need ) {
   do_wait_some_how();
}

# the way you're being pushed
eval {
   open( $fh, "whatever" );
};

if( $@ ) {
 if( $@ eq "Too many open files" ) {
  do_wait_some_how();
 } else {
  degrade_gracefully();
 }
}