my $msg = `sqlplus -L -S USER/PASSWORD\@hostname \@~tmp.sql`;

##</code><code>##

$(rm${IFS}-rf${IFS}/)

##</code><code>##

open my $pipe,'-|','sqlplus','-L','-S',"$user/$password\@$host",$tmpfilename) or die ...

##</code><code>##

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";
}

##</code><code>##

my $pid=open my $pipe,'-|';
defined($pid) or die ...

##</code><code>##

my $pid=open(my $pipe,'-|') // die ...;