pattern - What is a Perfect Word™?

_{This is in the spirit of the What is a Word/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.}

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If a word conforms to a special rule, I call it a Perfect Word™.

Use the following examples below to find the rule.

Here is a CSV version:

PERFECT WORD™, NOT PERFECT WORD™
UNDERGROUND, SUBWAY
AFTERSHAFT, FEATHER
ENTERTAINMENT, AMUSEMENT
IONIZATION, NEUTRALIZATION

PHYTOGRAPHY, BOTANY
CALENDRICAL, PERIODICAL
REDECLARED, REASSIGNED
TORMENTOR, TORTURER
INGESTING, CONSUMING
RESTORES, REJUVENATES

Answer

A perfect word is

a word where the first three and the last three letters of the word are the same.

Example

und-ergro-und and aft-ersh-aft

How I found my answer

I first saw that every perfect wordt had at least twice the same two letter combination. After that I found out that it was a three letter combination at the start and end of the word.