As pointed out you need to escape the ampersand in 'Joe & Jane' but you do not want to escape the other ampersand in 'CID&f6'. Other characters may also need escaping so best to use a module:

use URI;

my $uri = URI->new('www.mysite.com/cgi-bin/customer_info_get.pl');
$uri->query_form(
    f5 => 'CID',
    f6 => 'Joe & Jane',
);
print "$uri";
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Output:

www.mysite.com/cgi-bin/customer_info_get.pl?f5=CID&f6=Joe+%26+Jane
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