You can set $dbh->{Warn} to 0 while executing the table creation statements. This has the drawback that other (possibly important) database error messages are also suppressed. So a better strategy can be do redefine $SIG{__WARN__} to only suppress specific warnings:

my $tmpwarn = $SIG{__WARN__};
$SIG{__WARN__} = 
  sub { print STDERR @_ if $_[0] !~ m/NOTICE:  CREATE TABLE/; };
$sth->execute();
$SIG{__WARN__} = $tmpwarn;
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Or another option would of course be to redirect STDERR to a log file entirely.