Some funny code I wrote yesterday (the technique can be probably used for your problem too):

use Symbol qw(qualify_to_ref);
use overload;
my $stashref = qualify_to_ref("${namespace}::");

package tmp;
local *tmp:: = *$stashref;
overload->import('+' => sub {
     my $self = shift;
     my ($other, $info) = @_;
     # do some stuff
});
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This sets up overloaded operators for the namespace with the computed name $namespace.

Note that you cannot use "main" instead of "tmp", because the main namespace seems to be too read-only for this kind of hack (you'd get the error "Modification of read-only value attempted")