I am trying to figure out how perl determines the order in which to output STDOUT and STDERR text, both to a terminal and to a file. Consider the following code: 



#!/tool/bin/perl -w
use strict;

my $i = 0;
my @array;


for ($i = 0; $i < 100; $i++)
{
    printf("element $i of array = ");
    printf("%s\n", $array[$i]);
}

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If I run this code in a terminal, the output is as follows: 



> ./stderr_test.pl
Use of uninitialized value in printf at ./stderr_test.pl line 11.
element 0 of array =
Use of uninitialized value in printf at ./stderr_test.pl line 11.
element 1 of array =
Use of uninitialized value in printf at ./stderr_test.pl line 11.
element 2 of array =
Use of uninitialized value in printf at ./stderr_test.pl line 11.
element 3 of array =
Use of uninitialized value in printf at ./stderr_test.pl line 11.
element 4 of array =
...

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As you can see, the stdout text is interleaved with the stderr, which is what I would expect based on the order of the printf statements. If I redirect the text to a file however, the result is different: 



> ./stderr_test.pl > & log

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where log contains: 



Use of uninitialized value in printf at ./stderr_test.pl line 13.
Use of uninitialized value in printf at ./stderr_test.pl line 13.
Use of uninitialized value in printf at ./stderr_test.pl line 13.
...
element 0 of array = 
element 1 of array = 
element 2 of array = 
...

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As you can see, in this case perl first outputs all the stderr output, then outputs all the stdout output. Why does it do this?



Why is the behavior different when I output to a file vs. the terminal?



More importantly, how can I force the file output to appear in the correct order? Debugging is much easier when printed statements appear in programatic order.



I tried adding the following to my code above: 



open(STDERR,'>&', STDOUT);

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but that didn't make any difference in my output file.

In reply to ordering of stdout and stderr outputs by Special_K

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