Fix Assignment to $[ is no longer supported errors

[twwhatever]

Assignment to $[ is no longer supported in Perl 5.30. This at least allows the example to compile and work.

Test plan
Verify wbd.bin can be compiled to the same binary as the one in repo.

ldbsrc % rm ldb/wbd.bin
ldbsrc % git status
On branch master
Your branch is up to date with 'origin/master'.

Changes not staged for commit:
(use "git add/rm ..." to update what will be committed)
(use "git restore ..." to discard changes in working directory)
deleted: ldb/wbd.bin

no changes added to commit (use "git add" and/or "git commit -a")
ldbsrc % touch wbd/wbd.lex.utf8
ldbsrc % make -f Makefile.gnu lang=wbd all
fa_build_conf
--in=wbd/ldb.conf.small
--out=wbd/tmp/ldb.mmap.small.txt
fa_fsm2fsm_pack --type=mmap
--in=wbd/tmp/ldb.mmap.small.txt
--out=wbd/tmp/ldb.conf.small.dump
--auto-test
fa_build_lex --dict-root=. --full-unicode --in=wbd/wbd.lex.utf8
--tagset=wbd/wbd.tagset.txt --out-fsa=wbd/tmp/wbd.rules.fsa.txt
--out-fsa-iwmap=wbd/tmp/wbd.rules.fsa.iwmap.txt
--out-map=wbd/tmp/wbd.rules.map.txt
fa_fsm2fsm_pack --alg=triv --type=moore-dfa --remap-iws --use-iwia --in=wbd/tmp/wbd.rules.fsa.txt --iw-map=wbd/tmp/wbd.rules.fsa.iwmap.txt --out=wbd/tmp/wbd.fsa.small.dump
fa_fsm2fsm_pack --alg=triv --type=mmap --in=wbd/tmp/wbd.rules.map.txt --out=wbd/tmp/wbd.mmap.small.dump --auto-test
fa_merge_dumps --out=ldb/wbd.bin wbd/tmp/ldb.conf.small.dump wbd/tmp/wbd.fsa.small.dump wbd/tmp/wbd.mmap.small.dump
ldbsrc % printf "Hi There! This is a simple test." | fa_lex --ldb=ldb/wbd.bin --tagset=wbd/wbd.tagset.txt
Hi/WORD There/WORD !/WORD This/WORD is/WORD a/WORD simple/WORD test/WORD ./WORD
ldbsrc % git status
On branch master
Your branch is up to date with 'origin/master'.

nothing to commit, working tree clean