Often, when coding scripts, I encounter situations where I need to test to see if a variable matches an item that's stored in a list of values.

if (/$hash{$whatever}/) {
    do something;
}
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Assuming I'm thinking correctly so far, my next question has to do with creating the hash value. Is there anything "wrong" with populating a hash with keys and values that are the same? For example:

%SampleHash (
    'whatever' => 'whatever',
    'whoever' => 'whoever',
)
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The idea to use a hash to check if an item can be found in a set, is a good one. What is wrong with your last approach, is that if the string evaluates to false, a simple

if($SampleHash{$string}){ ... }
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will fail to see it. The strings that do that, are "" and "0".

I ordinarily use one of the following approaches:

foreach(LIST) {
    $checked{$_}++;
}

if($checked{$string}) { ... }
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or

@checked{LIST} = ();

if(exists $check{$string}) { ... }
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but there are more. Use what seems most appropriate at the time.