(1) I don’t just pretend, I use use strict;.

(2) Yeah, I forgot my before definition of $dsn; $dbh, my mistake. However, I only copy-pasted this code, I have properly defined all variables in the full code. On the other hand, I modified the code so much that I cannot reproduce the error now. Still I have some other errors (like Global symbol "$cols_type" requires explicit package name at ./test6.pl line 156.), which are somewhat related to this, however, all my variables are properly declared/defined (I have double-checked it).

I’m sorry I opened this poor-quality question. :(