OOP Perl

#!/usr/bin/env perl

use v5.14;
use strict;
use warnings;
use Data::Dumper;

my $wbuf=3;
my $wtr;
print "start $wbuf\n";

$wtr = sub {
    my $done_cb = shift;
    my $mbuf=$wbuf;
    print "done_cb2 ".Dumper($done_cb);
    print "wtr3 ".Dumper($wtr);
    $wbuf--;
    print "cb $wbuf $mbuf\n";
    if ( $wbuf ) {
        print "cb wbuf\n";
        $wtr->($done_cb);
    } else {
        print "cb undef\n";
        print "wtr4 ".Dumper($wtr);
        undef $wtr;
        print "wtr5 ".Dumper($wtr);
        $done_cb->();
        print "wtr7 ".Dumper($wtr);
        print "after exit\n";
    }
    print "return wtr\n";
    print "cb $wbuf $mbuf\n";
};

print "wtr1 ".Dumper($wtr);

$wtr->(
    sub {
        my $done_cb = shift;
        print "done_cb6 ".Dumper($done_cb);
        print "exit $wbuf\n";
    }
);

print "stop\n";