So - the documented examples between the DBD and DBI modules conflict.

However, the code in DBI.pm resolves this conflict - it says:

sub connect {
 ....
 # extract dbi:driver prefix from $dsn into $1
    $dsn =~ s/^dbi:(\w*?)(?:\((.*?)\))?://i
            or '' =~ /()/; # ensure $1 etc are empty if match fails
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so - it will accept EITHER case for the "DBI" part.

The part about "(Letter case is significant.)" applies to the DBD driver name, not the the "DBI" or "dbi" - it will even accept "DbI".