I like this approach, too; the hash makes exceptions to the "uppercase bracket" basic rule perfectly clear. However, I also agree with other posters (1) treating $country as a temporary variable isn't great practice, and (2) it's not entirely clear what $card->country is allowed to return. Given those concerns, my own preference would be

my %exceptions = ('' => '', gbr => '');
my $c = $card->country || '';
my $country = exists($exceptions{$c}) ? $exceptions{$c} : uc "[$c]";
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