Take a look at threads. If you are using AS, then the threads module is locate under the "Pragmas" section of the "Table of Contents" in the left-hand frame of the html docs.

If all you want to do is run the sub standalone to completion, then that is as simple as

use threads;

sub YourSub {
    ...
    return $result;
}

...

my $thread = threads->new( \&YourSub, $arg1, $arg2, ... );

# If your not interested in  the result from the sub then
# $thread->detach;
....
# Otherwise, this will retrieve the return value.
# Note: This will block until the thread finishes.
# One (of several) things missing from the API as it stands
# is any way to determine if the thread has finished 
# without blocking.
my $result = $thread->join;
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If you need to communicate between your main code and the subs whilst it is running, then life gets a little more complicated.

If you give a clearer picture of what the sub is going to be doing (like the code:), then it may be possible to give a better example.