See the current Perl documentation for Thread::Semaphore.
Here is our local, out-dated (pre-5.6) version:
Thread::Semaphore - thread-safe semaphores
my $s = new Thread::Semaphore;
$s->up; # Also known as the semaphore V -operation.
# The guarded section is here
$s->down; # Also known as the semaphore P -operation.
Semaphores provide a mechanism to regulate access to resources. Semaphores,
unlike locks, aren't tied to particular scalars, and so may be used to
control access to anything you care to use them for.
Semaphores don't limit their values to zero or one, so they can be used to
control access to some resource that may have more than one of. (For
example, filehandles) Increment and decrement amounts aren't fixed at one
either, so threads can reserve or return multiple resources at once.
- new NUMBER
new creates a new semaphore, and initializes its count to the passed number. If
no number is passed, the semaphore's count is set to one.
- down NUMBER
down method decreases the semaphore's count by the specified number, or one if
no number has been specified. If the semaphore's count would drop below
zero, this method will block until such time that the semaphore's count is
equal to or larger than the amount you're
downing the semaphore's count by.
- up NUMBER
up method increases the semaphore's count by the number specified, or one if
no number's been specified. This will unblock any thread blocked trying to
down the semaphore if the
up raises the semaphore count above what the
downs are trying to decrement it by.