In the TMTOWTDI spirit, if your keys are really going to be "1", "2", "3", "4", and so on, you can use an array:

my @lookup;
@lookup[1..4] = (
    567898,
    543267,
    234565,
    987654,
);

my @test = qw(0 a 2 1);
for (@test) {
    print defined $lookup[$_] ? $lookup[$_] : $_, "\n";
}
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