This seems to work fine for me:

#!/usr/bin/env perl

use v5.14;

package The::Package {
    use Carp;
    use base "Exporter";
    BEGIN { our @EXPORT = qw( my_generator foo takes_coderef ) };
    
    my $scratchpad;
    sub my_generator (&) {
        $scratchpad = {};
        shift->();
        my $tmp = $scratchpad;
        undef $scratchpad;
        return $tmp;
    }
    
    sub foo ($) {
        croak "foo must be called within my_generator" unless $scratch
+pad;
        $scratchpad->{foo} = shift;
    }
    
    sub takes_coderef (&) {
        croak "takes_coderef must be called within my_generator" unles
+s $scratchpad;
        $scratchpad->{coderef} = shift;
    }
    
    BEGIN { $INC{"The/Package.pm"} = __FILE__ };
};

use The::Package;
use Data::Dumper;

my $complex_result = my_generator {
    foo 'bar';
    takes_coderef { ... };
};

print Dumper($complex_result);

foo 'baz'; # Should not affect $complex_result.
           # Ideally, should croak, or not even compile.
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If you want to be able to nest calls to my_generator then you need to have a stack of scratchpads, but that's very little extra work.