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#!/usr/bin/perl
use Socket;
...

    close $sock;
}

$ perl ./aliasing1.pl
before aliasing: sock=5, STDIN=0, STDOUT=1
after aliasing:  sock=5, STDIN=5, STDOUT=5
^C
$

$ telnet localhost 8080
Trying 127.0.0.1...
...
foo
you typed: foo
Connection closed by foreign host.

$ perl -d ./aliasing1.pl

...
328:    sub DESTROY {}
  DB<1> q
$

POSIX::dup2(fileno($sock), 0) or die "Cannot dup sock to STDIN: $!";
POSIX::dup2(fileno($sock), 1) or die "Cannot dup sock to STDOUT: $!";

open STDIN,  "<&", $sock or die "Cannot dup sock to STDIN: $!";
open STDOUT, ">&", $sock or die "Cannot dup sock to STDOUT: $!";

#!/usr/bin/perl

...
              fileno($fh), fileno(STDOUT);

print "foo\n";

$ perl ./aliasing2.pl
fds before aliasing: fh=3, STDOUT=1
fds after aliasing:  fh=3, STDOUT=3

$ perl -d ./aliasing2.pl

...
  use o inhibit_exit to avoid stopping after program termination,
  h q, h R or h o to get additional info.  
  DB<1> q