What are you're doing doesn't look good for me. You can do the same much simpler:

my @buffer;

sub store {
    return "Buffer is full" if @buffer >= 4;
    push @buffer, $_[0];
}

sub retrieve {
    shift @buffer;
}
[download]

Update: ikegami is right, if you have reader and writer threads ring buffer is quite useful. And my code would require some synchronization.