sqlite_open_flags => "DBD::SQLite::OPEN_READONLY",
You're passing a string to sqlite_open_flags, while the documented way is an exported constant: 
use DBD::SQLite::Constants qw/:file_open/;
my $dbh = DBI->connect("dbi:SQLite:$dbfile", undef, undef, {
  sqlite_open_flags => SQLITE_OPEN_READONLY,
});

[download]
Experiment shows that it's currently 1, but we shouldn't be relying on that value. Following the documented way, I see DBD::SQLite trying to open the file read-only: 
$ strace -e trace=%file perl -MDBI -MDBD::SQLite::Constants=:file_open
+ -E'
 DBI->connect("dbi:SQLite:foo.sqlite", undef, undef, {
  sqlite_open_flags => SQLITE_OPEN_READONLY
 });
'
<...>
openat(AT_FDCWD, "REDACTED/foo.sqlite", O_RDONLY|O_NOFOLLOW|O_CLOEXEC)
+ <...>

[download]

In reply to Re: Making DBI with SQLite3 truly read-only by Anonymous Monk
in thread Making DBI with SQLite3 truly read-only by mldvx4

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