Interestingly enough, if you add use strict (at least on 5.8.9 and 5.10.1) to package foo, B::Deparse does not reveal all, like so:

package foo;
sub BEGIN {
    require strict;
    do {
        'strict'->import
    };
}
sub ARGV {
    use strict 'refs';
    print "in foo::ARGV\n";
}
package main;
use strict 'refs';
'foo'->ARGV;
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(in fact just making sub ARGV not the first line of code after package foo is enough to confuse B::Depase in this.