#!/usr/bin/perl -l

my $file = "file.txt";

print "before";
{
   print STDERR "fileno before local: ", fileno(STDOUT);

   open (my $old_stdout, '>&', *STDOUT) or die;
   close (STDOUT);
   open (STDOUT, '>', $file) or die;

   print STDERR "fileno after open: ", fileno(STDOUT);

   system("ls");

   close (STDOUT);
   open (STDOUT, '>&', $old_stdout) or die;
}

print "after";