I'd be explicit in the length and drop the trailing comma after your regex. How about letting the documentation read in the order of execution?

my @letter_seq = map(
         $curr->[STYLE].":".join(',', splice @letter, 0, length($_)),
         join('', @property{@letter}) =~ /\G(X|D+|L+)/g
);

# Join the result of a hash slice taken from the letter-to-property 
# mapping. Perform a match against the joined string to group together
# alphanumeric characters and leave all others distinct. Remove each m
+atch
# from the @letter array and join the character(s) to an output string
+ such
# that the resulting format is i.e., "some_type:f4,!".
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