It depends on how you are building the content of the "list box" (by which I assume you mean "An HTML select element").

Assuming you are using the built in methods of the CGI module (not the approach I would use, I'd much rather separate things out with Template-Toolkit), then just pass different data after testing for the condition. The approach is pretty much the same with any other method, it just needs adapting for where you pass the data to.

my $condition = 1;
my @options = qw/val1 val2/;

if ($condition) {
  @options = qw/val3 cal4/;
}

my $q = CGI->new();
print $q->Select(
        map { $q->option($_) } @options
);
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