use DBD::SQLite::Constants qw/:file_open/;
my $dbh = DBI->connect("dbi:SQLite:$dbfile", undef, undef, {
  sqlite_open_flags => SQLITE_OPEN_READONLY,
});

##</code><code>##

$ 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) <...>