perldoc -f local

That doesn't answer the OP's question.
Interestingly though, in the OP's code there are three lines of useful documentation telling us what it does.
The question that still needs to be answered is Why does it do it without explicitly setting it to = 0;

Update:
The answer is to be found more than just a little below the surface of   perldoc DBI   where it says:

Also note that in this older form of "connect", the "$dbh->{AutoCommit}" attribute is undefined

Still not that obvious.

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