I've narrowed it down to

perl -E'no autovivification; push @a, [exists(&no)]; '
Modification of a read-only value attempted at -e line 1.
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please note that the push is also needed to cause that error

as already explained, is exists returning an empty list - or something "similar" - for false in the buggy case.

perl -MData::Dump -E'dd exists(&non); no autovivification; dd exists(&
+non);'
""
()
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