From the original code, it's clear that $card->country could be "", and, if so, shouldn't get brackets. But you've come closest to how I'd like to think I'd do it (assuming $card->country is always at least defined):

my $country = uc("[" . $card->country . "]");
$country = '' if $country eq "[GBR]" || $country eq "[]";
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(This differs from the original if $card->country starts off as uppercase GBR, or if $card->country is 0 or undef or some overloaded object that acts false, but I'm guessing that's a good thing.)