#!/usr/bin/perl -w
# show two concurrent processes intermingling their STDERR to the same file.
# Name the program 'test_err.pl' and invoke it from command line with:
#    test_err.pl hs

use strict;

my $err_file = 'errfile.txt';
open STDERR, ">> $err_file"
    or die "cannot open initial $err_file:  $!\n";

$SIG{'__WARN__'} = sub { syswrite STDERR, $_[0] };

my $what_to_do = shift;
warn "$what_to_do hello world\n";

system( 'test_err.pl', 'abc' )
    if $what_to_do eq 'hs';

my $test_string;
warn "success\n" if $test_string eq 'xyz';

warn "$what_to_do goodbye world\n";

# Adds the following text to 'errfile.txt':
__DATA__
hs hello world
abc hello world
Use of uninitialized value in string eq at C:\aa\test_err.pl line 19.
abc goodbye world
Use of uninitialized value in string eq at C:\aa\test_err.pl line 19.
hs goodbye world