use strict;
use warnings;

package Foo;

sub new {
    my ($class, $callback) = @_;

    return bless {callback => $callback}, $class;
}

sub doIt {
    my ($self) = @_;

    return $self->{callback}->();
}


package main;

my $test = "Big test";
my $foo = Foo->new (sub {return hook (\$test);});

$foo->doIt ();

print $test;


sub hook {
        my ($test, $filename, $buffer, $bytes_read, $data) = @_;
        $$test .= ': success';
}