I'm not sure why you're using the string "Undef" instead of an actual undef, maybe that could be changed wherever you are generating %TableHeaders? Anyway, given what you've showed, here's a suggestion, this does an lc so the casing of "Undef" shouldn't matter:

    -choices => [
        map { { -name=>$TableHeaders{$_}, -value=>$_ } }
        grep { length $TableHeaders{$_}
            && lc($TableHeaders{$_}) ne 'undef' }
        sort keys %TableHeaders ],
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If the keys of %TableHeaders are always numbers, you should do ... sort {$a<=>$b} keys ... instead.