#!/usr/bin/perl -l

my $file = "file.txt";

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

   local *STDOUT;
   open (STDOUT, '>', $file) or die;

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

   system("ls");
}

print "after";

##</code><code>##

before
fileno before local: 1
fileno after open: 3
740300.pl
after

##</code><code>##

#!/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";

##</code><code>##

$ perl 740300.pl
before
fileno before local: 1
fileno after open: 1
after

$ cat file.txt
740300.pl

##</code><code>##

#!/usr/bin/perl -l

use IPC::Open3 qw( open3 );

my $file = "file.txt";

print "before";

{
   open (my $fh, '>', $file) or die;
   my $pid = open3('<&STDIN', '>&'.fileno($fh), '>&STDERR', 'ls');
   waitpid($pid, 0);
}

print "after";