my $file = "file.txt";

print "before\n";
{
   open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
   open (STDOUT, ">$file") or die;
   system("ls");
   open STDOUT, ">&", $oldout  or die "Can't dup \$oldout: $!";
}

print "after\n";