I can replicate your problem:

foo.pl:

#!/usr/bin/perl

use strict;
use warnings;

use XML::XSLT         qw( );
use Object::Destroyer qw( );

my $xslt = Object::Destroyer->new(
    XML::XSLT->new('foo.xslt', warnings => 1),
    'dispose',
);

$xslt->transform('foo.xml');
print $xslt->toString;
[download]

foo.xml:

<?xml version="1.0" encoding="utf-8"?>
<PLMXML xmlns="http://www.plmxml.org/Schemas/PLMXMLSchema">
  <Form id="id24" subType="Part Revision Master">
    <UserData>
      <UserValue title="cage_code" value="98747"/>
    </UserData>
  </Form>
</PLMXML>
[download]

foo.xslt:

<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Tr
+ansform">
<xsl:template match="/">
<xsl:value-of select="/PLMXML/Form[@subType='Part Revision Master']/Us
+erData/UserValue[@title='cage_code']/@value"/>
</xsl:template>
</xsl:stylesheet>
[download]

Also tried:

Works:        "/PLMXML/Form[@id='id24']/UserData/UserValue[@title='cag
+e_code']/@value"
Doesn't work: "/PLMXML/Form[@subType='Part Revision Master']"
Doesn't work: "/PLMXML/Form[@subType="Part Revision Master"]
+/UserData/UserValue[@title='cage_code']/@value"
[download]

(This should have been in your original post)