Extra xml tag when using Lib:XSLT to create text file - perl

Question

Cross-post http://perlmonks.org/index.pl?node_id=979710
I'm trying to create a text file from some XML using Perl and Lib::XSLT, my transformation works fine except Lib::XSLT adds an unwanted ?xml version tag to the start of the file, how can I stop it doing this?
Here's my XSLT:
<xslt:stylesheet version="1.0" xmlns:data="http://www.SDMX.org/resources/SDMXML/schemas/v2_0/generic" xmlns:xslt="http://www.w3.org/1999/XSL/Transform" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:message="http://www.SDMX.org/resources/SDMXML/schemas/v2_0/message" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="text" omit-xml-declaration="yes"/>
<xslt:param name="sep">|</xslt:param>
<xslt:output method="text" />
<xslt:template match="message:MessageGroup">
<xslt:for-each select="data:DataSet">
<!-- get dimensions (but not time) and store in dimensions variable -->
<xslt:for-each select="data:Series">
<xslt:variable name="dimensions">
<xslt:for-each select="data:SeriesKey">
<xslt:for-each select="data:Value">
<xslt:value-of select="#value" />
<xslt:value-of select="$sep" />
</xslt:for-each>
</xslt:for-each>
</xslt:variable>
<!--get obs statuses and store in obs statuses variable-->
<xslt:variable name="obsStatuses">
<xslt:for-each select="data:Attributes">
<xslt:for-each select="data:Value">
<xslt:value-of select="#value" />
</xslt:for-each>
</xslt:for-each>
</xslt:variable>
<!--write out dimensions variable, time, observation, obsstatuses variable-->
<xslt:for-each select="data:Obs">
<xslt:value-of select="$dimensions" />
<xslt:value-of select="data:Time" />
<xslt:value-of select="$sep" />
<xslt:value-of select="data:ObsValue/#value" />
<xslt:value-of select="$sep" />
<xslt:value-of select="data:Attributes/data:Value/#value"/>
<xslt:text>
</xslt:text>
</xslt:for-each>
</xslt:for-each>
</xslt:for-each>
</xslt:template>
</xslt:stylesheet>
Here's the Perl:
use Lib::XSLT;
my $parser = XML::LibXML->new();
my $xslt = XML::LibXSLT->new();
my $source = XML::LibXML->load_xml(location => "$xmlFile");
my $style_doc = $parser->parse_file(Path::Class::File->new("$xsltFile"));
my $stylesheet = $xslt->parse_stylesheet($style_doc);
open OUTPUTFILE, ">>$outputFile" or die("Unable to open $outputFile, $!");
print OUTPUTFILE $stylesheet->transform($source);
close OUTPUTFILE;

Answer 1

Storing the result of $stylesheet->transform() and using $stylesheet->output_file() fixes this issue, e.g:
use Lib::XSLT;
my $parser = XML::LibXML->new();
my $xslt = XML::LibXSLT->new();
my $source = XML::LibXML->load_xml(location => "$xmlFile");
my $style_doc = $parser->parse_file(Path::Class::File->new("$xsltFile"));
my $stylesheet = $xslt->parse_stylesheet($style_doc);
my $results = $stylesheet->transform($source);
$stylesheet->output_file($results, $outputFile);

Answer 2

Why is the <?xml> declaration unwanted? It is valid XML and and has no influence on parsers.