Open a file handle on a string:

use strict;
use warnings;
use XML::Writer;

my $outputStr;

open my $output, '>', \$outputStr;

my $writer = new XML::Writer(OUTPUT => $output);

$writer->startTag("greeting", "class" => "simple");
$writer->characters("Hello, world!");
$writer->endTag("greeting");
$writer->end();
close $output;

print $outputStr;
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Prints:

<greeting class="simple">Hello, world!</greeting>
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Having captured the output to a string you can print where ever you want and as many times as you want.

Update helps to copy and paste the right stuff!