Well, since fork is essentially just a disguised threads->create, the simplist mechanism is to use a shared variable. This is simplistic, but it demonstrates the principle.

#! perl -slw
use strict;
use threads;
use threads::shared;

my $signal :shared = 0;

my $parent = $$;
print "$$:\tI am the parent.\n";

unless( my $pid = fork){
    print "$$:\tI am the child.\n";
    lock $signal;
    $signal = 1;
    exit;
}

print "$$:\tWaiting for the child to signal.\n";
sleep 1 until $signal;
print "The child signalled.";

__END__
c:\test>junk2
1124:   I am the parent.

1124:   Waiting for the child to signal.

-1912:  I am the child.

The child signalled.
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