Frankly, I'm surprised that this much works this well.

As for the synchronization issue, the canonical way to solve it in Go is with a sync.WaitGroup, like this:

package main

import (
    "C"
    "fmt"
    "os"
    "sync"
)

func main() {}

var wg sync.WaitGroup

//export Yaph
func Yaph() {
    fmt.Println("Every day I get in the queue")
    fmt.Println("To get on the bus that takes me to you")
}

//export Acme
func Acme() {
    wg.Add(1)
    go func() {
        defer wg.Done()
        fmt.Println("I guess i'm a closure")
        fmt.Println (os.Getpid())
    }()
}

//export Wait
func Wait() {
    wg.Wait()
    fmt.Println("All async goroutines have terminated")
}
[download]

Then, in the calling perl code:

#!/usr/bin/env perl

use strict;
use warnings;
use FFI::Platypus;

use feature qw ( say );

say qq($0 $$);

my $ffi = FFI::Platypus->new( api => 1 );

$ffi->lib('./yaph.so');

$ffi->attach(Yaph => []);

$ffi->attach(Acme => []);

$ffi->attach(Wait => []);

Yaph();

Acme() for 1..300;

Wait();

END  {
    say qq(Too much, Magic Bus!) for 1 .. 3;
}

__END__
[download]

This has the disadvantage of relying on a global variable on the Go side. I don't see any way of returning a waitgroup variable created on the fly from an exported Go function to be encapsulated into a Perl variable, then feed it back into a second Go function. Nor do I see any other way to influence the Go runtime from the Perl side.

Still, it's an impressive demonstration. Real multithreading in a Perl program! :P