[ Duplicate post. Please ignore ]

Use a trie instead of a hash table.

For future reference,

    if(exists $wordCount{$_}) {
        $wordCount{$_} = $wordCount{$_} + 1;
    }
    else {
        $wordCount{$_} = 1;
    }
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can be shortened to

    ++$wordCount{$_};
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It won't help your memory problem, just your code clarity.