First, do you get an error message? You can generate them in 2 ways:

# Ask for errors at database handle creation
my $dbh2 = DBI->connect(
  $dsn, $user, $password,
  { RaiseError => 1, AutoCommit => 0 }
);

# Or check your method call
$dbh2->do($sql) or die "Cannot execute SQL '$sql': $DBI::errstr;
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Those error messages are often helpful. Were I to guess, your problem is that you've used the wrong kind of quotes. Try

  $dbh2->do(
    qq{alter session set nls_timestamp_format = 'YYYY-MM-DD HH24:MI:SS
+'}
  ) or die "Can't set timestamp format: $DBI::errstr";
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