It does recurse through all 8-letter combos, but short-circuits at each level if the combination so far cannot be derived from the dictionary supplied.

        next unless exists $tree->{ $_ };
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If the letter combination for your 'eehnortw' existed in the dictionary, it would be found very quickly:

[17:25:23.81] C:\test>type hdb.dict
exactest
one
two
three
four
five
six
seven
eight
nine
ten
torewhen

[17:25:47.91] C:\test>1056884-calc hdb.dict
0.00380706787109375

[5] eehnortw : torewhen three one two ten
[1] aceesttx : exactest
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Comment out the short-circuit and it will consider all 8-letter possibilities .. and run more slowly.