my $pid=open my $pipe,'-|' or die ...;
if ($pid) {
    # parent, see perlipc
} else {
    # child process
    # STDOUT is connected to $pipe
    # STDERR is inherited from the parent process
    open STDERR,'>&STDOUT' or die ...;
    # now STDERR is also connected to $pipe
    open STDOUT,'>','/dev/null' or die ...;
    # now STDOUT is discarded
    exec 'sqlplus','-L','-S',"$user/$password\@$host",$tmpfilename);
    die "exec failed";
}