use warnings;
use strict;

{
	package Tie::Scalar::Callbacks;
	sub TIESCALAR {
		my $class = shift;
		my %self;
		@self{qw/ val store_cb fetch_cb /} = @_;
		return bless \%self, $class;
	}
	sub FETCH {
		my $self = shift;
		$self->{fetch_cb}->($self->{val}) if $self->{fetch_cb};
		return $self->{val};
	}
	sub STORE {
		my $self = shift;
		my $val = shift;
		$self->{store_cb}->($self->{val}, $val) if $self->{store_cb};
		$self->{val} = $val;
	};
	sub DESTROY { %{shift()}=() }
}

tie my $name, 'Tie::Scalar::Callbacks', "foo",
	sub { print "storing <$_[1]>, was <$_[0]>\n" };
print "name is $name\n";
$name = "bar";
print "name is $name\n";

__END__

name is foo
storing <bar>, was <foo>
name is bar