{
my $returned_data = [ ];

sub my_fetch_callback { push @{ $returned_data }, $_[0] }

routine_wanting_callback( \&my_fetch_callback );

for my $datum ( @{ $returned_data } ) {
  munge_it( $datum );
}
}

##</code><code>##

my $line_holder = Line::Holding->new( );

routine_wanting_callback( sub { $line_holder->hold( $_[0] ) } );

$line_holder->process_held_lines;