#!/usr/bin/perl -Tw

open STDIN, '/dev/null'   or die "Can't read /dev/null: $!";
open STDOUT, '>/dev/null' or die "Can't write to /dev/null: $!";
open STDERR, '>/tmp/log'  or die "Can't write to /tmp/log: $!";

my $oldfh = select STDERR;
local $| = 1;
select $oldfh;
warn "started\n";
# do something time-consuming
sleep 1, warn "$_\n" for 1..20;
warn "completed\n";