non-blocking reads from the cookbook.

use Fcntl;

sysopen(MODEM, "/dev/cua0", O_NONBLOCK|O_RDWR)
    or die "Can't open modem: $!\n";
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be warned : once the file handle's non blocking, statements that try to switch back will return undef and set $!.

--
Hope this helps -- my OS doesn't have any of this fancy blocking or nonblocking hoo-hah, so I can't test properly ;-)