No need for globals, just have the coderef use lexicals in the scope it's declared in (i.e. use a closure).

{
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 );
}
}
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Update: Moved bracket after for so it would, you know, actually work. Duurr. Stupid trees making pollen affecting sinuses with their pollen making brane numbing congestion stuff.

Another example: You could also pass a coderef which stores the results by calling methods on an object you provide.

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

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

$line_holder->process_held_lines;
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Nothing says the callback has to call a named subroutine sitting in the symbol table (unless the callback wanting routine says it takes the name of a subroutine of course; but that'd just be bad design :).