$board =~ /(?<!\w).{2,}(?!\w)(?{ push @pat, [ $-[0], $& ] })(*FAIL)/
+;
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This is the heart of finding a place to move. For a word to be a legal move, it must match one of the patterns in @pat. The (?<!\w) prevents putting the new word next to a preceding word, and the (?!\w) prevent putting the new word abutting a following word. The(*FAIL) forces the regex engine to try every possible place to find a match.

Try

perl -le ' "abcd" =~ /.{2,}(?{print $&})(*FAIL)/ '
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and notice it prints out every substring with two or more characters.