I hope I have interpreted your question correctly, if not apologies.
You can find out if the hash exists with: if defined. However, you might consider another way of doing this. I suggest you create a single hash where the keys are the dates. The value to each key would be a reference to an anonymous hash, like this:

$hash{$date} = {};
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To "play with the hash", adding values would be:

$hash{$date}->{key} = value;
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To see if the hash exists, use exists:

if (exists $hash{$date}) {...}
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