I have refactored into the following and it does not fail.

use WWW::Mechanize qw();

my $browser = WWW::Mechanize->new(
    autocheck => 1,
);
$browser->get('https://shop.safeway.com/brands/swy/register/signin.asp
+?from=/superstore');

$browser->submit_form(
    with_fields => {
        pzipcode    => 90210,
    },
);
[download]

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