use strict;

package Foo;
use Carp;

sub new {
   my ($class,$msg) = @_;
   my $self = {msg => $msg};
   return bless $self,$class;
}

sub print_me {
   my ($self,$cb) = @_;
   croak "not a code ref" unless ref $cb eq 'CODE';
   return $cb->($self->{'msg'});
}

package main;

my $foo = Foo->new('hello world');

print $foo->print_me(sub {ucfirst shift}), "\n";