add xml:id to item and target to ref

convert.pl

#!/usr/bin/perl

use 5.012;
use warnings;

binmode STDOUT, ':utf8';

use XML::LibXML;
use XML::LibXSLT;

my $xslt   = XML::LibXSLT->new;
my $parser = XML::LibXML->new;
my $xpc = XML::LibXML::XPathContext->new;
$xpc->registerNs( 't', 'http://www.tei-c.org/ns/1.0' );

my $source = $parser->load_xml( location => shift );
my $xsl; $xsl = do { local $/; <DATA> };
my $style_doc = XML::LibXML->load_xml( string => $xsl );
my $stylesheet = $xslt->parse_stylesheet($style_doc);

$stylesheet->register_function( 'urn:p', 'id', sub {
    my ($s) = @_;
    $s = "$s";
    $s =~ /(\d+)/;
    return $1 ? "i-$1" : "i-$s-XXX";
});

$stylesheet->register_function( 'urn:p', 'ref', sub {
    my ($s) = @_;
    $s = "$s";
    $s =~ /(\d+)/;
    return $1 ? "#i-$1" : "#i-$s-XXX";
});

my $results = $stylesheet->transform( $source );
my $xml = $stylesheet->output_as_chars($results);
say $xml;

__END__
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:t="http://www.tei-c.org/ns/1.0"
  xmlns:p="urn:p"
  exclude-result-prefixes="t p"
  xpath-default-namespace="http://www.tei-c.org/ns/1.0"
>

<xsl:output method="xml" version="1.0" indent="no" encoding="UTF-8"/>

<xsl:template match="t:item">
  <xsl:element namespace="http://www.tei-c.org/ns/1.0" name="item">
    <xsl:attribute name="xml:id">
      <xsl:value-of select="p:id(*/t:label/text())"/>
    </xsl:attribute>
    <xsl:apply-templates select="@*|*|text()|processing-instruction()|comment()"/>
  </xsl:element>
</xsl:template>

<xsl:template match="t:ref">
  <xsl:element namespace="http://www.tei-c.org/ns/1.0" name="ref">
    <xsl:attribute name="target">
      <xsl:value-of select="p:ref(text())"/>
    </xsl:attribute>
    <xsl:apply-templates select="@*|*|text()|processing-instruction()|comment()"/>
  </xsl:element>
</xsl:template>

<xsl:template match="@*|*|text()|processing-instruction()|comment()" priority="-1">
  <xsl:copy>
    <xsl:apply-templates select="@*|*|text()|processing-instruction()|comment()"/>
  </xsl:copy>
</xsl:template>

</xsl:stylesheet>