By way of demonstration. Looking up 9000 words in a hash is 6X faster than splitting those same 9000 words out of a string:

 $s = 'the quick brown fox jumps over the lazy dog' x 1000;;
++$h{ $_ } for qw[ the quick brown fox jumps over the lazy];;

cmpthese -1,{
    a => q[ my @words = split ' ', $s;],
    b => q[ 
        my $n=0; 
        $h{$_} && ++$n for (qw[ 
             the quick brown fox jumps over the lazy dog
        ]) x 1000; 
    ]
};;
    Rate    a    b
a 95.4/s   -- -85%
b  644/s 575%   --
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