#!/usr/bin/perl -w

my $output;
print "first\n";
open (FH, '>', \$output);
print FH "Hello world\n";
close(FH);
$output=undef;
print "second\n";
open (FH, '>', \$output);
print FH "Hello world\n";
close(FH);
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Output:

first
second
Use of uninitialized value in open at o4.pl line 10.
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Would not it be easier to open() the scalar and select() the handle?

open my $FH, '>', \$output;
my $old_FH = select($FH);

print "Hello world\n";

select($old_FH);
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Thanks for the suggestion!

I actually don't know whether the code whose output I want to capture specifically prints to STDOUT (I didn't write it). I thought of the select solution too, but then I thought, "but what if it writes directly to STDOUT?" So I did this.

The solution I had actually works but produces the warning. I mention the "real" problem just in case I'm doing something brainless.

    my $output = '';
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 my $output; # this value is unintialized
 open my $old_stdout, '>&=', \*STDOUT;
 close STDOUT;
 # Use of uninitialized value in open at the following line
 open STDOUT, '>', \$output;
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So it actually spits a warning the first time.

Where does that warning go? It's not on my screen when I run it. As I see it, $output is uninitialized both times, but the warning is not printed the first time.

I'm confused also because what I pass to open is a reference to $output which is always a real value regardless of whether $output is initialized or not.

I see where I went wrong.

To give a more concise answer to this question, I got the code to run and saw the results just as you reported. So I started experimenting.

I noticed there are a lot of operations on references happening here, which makes me leery when you're mucking with global symbols (STDOUT, for instance). So I took $output and gave it persistence in scope:

 our $output ; # changed 'my' to 'our'
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The problem went away.

By the way, I agree with the monks who suggested using select().

Peace, monks.

Bro. Doug :wq

The issue seems real. Consider:

use strict;
use warnings;

wibble ();
wibble ();

sub wibble {
    my $unicode16Str;
    
    open my $UTF, '>:encoding(UTF-16LE)', \$unicode16Str;
    print $UTF "Wibble";
    close $UTF;
}
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Generates:

Use of uninitialized value in open at noname1.pl line 10.
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for the second call. However changing my $unicode16Str; to my $unicode16Str = '';

runs clean. Making $unicode16Str global to the sub is also fine. A closure issue perhaps?

    open my $UTF, '>:encoding(UTF-16LE)', \$unicode16Str;
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That may be a completely different issue.

I don't think so. The warning is only issued for the second open. The code otherwise works as expected. Initialising the output buffer ($unicode16Str) resolves the issue. Making the output buffer global also resolves the issue.

---

DWIM is Perl's answer to Gödel

#!/usr/bin/perl -w
use strict;
foreach (0..1) {
  my $scalar;
  open my $h, ">", \$scalar;
  print "End of $_\n";
}
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