XML::Writer blesses $doc for some reason. I presume IO::Compress::gzip checks for that even though it would have no effect in this case. Copying $doc doesn't copy the blessing. 

use strict;
use warnings;

use Devel::Peek  qw( Dump );
use Scalar::Util qw( blessed );
use XML::Writer  qw( );

my $writer = new XML::Writer(OUTPUT => \my $doc);
$writer->startTag("root");
$writer->startTag("foo");
$writer->endTag("foo");
$writer->endTag("root");
$writer->end();

Dump $doc;
print(blessed(\$doc)?1:0, "\n");   # 1

my $copy = $doc;

Dump $copy;
print(blessed(\$copy)?1:0, "\n");  # 0

[download]
SV = PVMG(0x18c5614) at 0x182a39c
  REFCNT = 2
  FLAGS = (PADMY,OBJECT,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x1922fbc "<root><foo></foo></root>\n"\0
  CUR = 25
  LEN = 28
  STASH = 0x19154b4     "XML::Writer::_String"
1
SV = PVMG(0x18c5654) at 0x1915784
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x1925184 "<root><foo></foo></root>\n"\0
  CUR = 25
  LEN = 28
0

[download]

In reply to Re: XML::Writer oddness by ikegami
in thread XML::Writer oddness by TedHopp

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