Update xml file containing namespace prefix with XML::Twig

Question

I have the following information in an xml file I need to amend:

<?xml version="1.0" encoding="utf-8"?>
<x:workbook xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships" 
            xmlns:x="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
    <x:workbookPr codeName="ThisWorkbook"/>
    <x:bookViews>
        <x:workbookView firstSheet="0" activeTab="0"/>
    </x:bookViews>
    <x:sheets>
        <x:sheet name="Sheet1" sheetId="2" r:id="rId2"/>
    </x:sheets>
    <x:definedNames/>
    <x:calcPr calcId="125725"/>
</x:workbook>

In general, the prefix "x:" can be anything, including the empty prefix.

I now need to add a child element "x:sheet" to the "x:sheets" element in the file. However, if the file contents had no prefixes then I would need to add a child element "sheet" to the "sheets" element in the file.

Since the prefix, in general, can be anything I use the following perl code to scan and amend the file.

use strict;
use warnings;
use XML::Twig;

my $fileName1='/folder1/file1.xml';
my $fh1;
my $fh1_filename='/folder1/file1_NEW.xml';
my $tw1=new XML::Twig(
map_xmlns => {
    'http://schemas.openxmlformats.org/officeDocument/2006/relationships' => 'r',
    'http://schemas.openxmlformats.org/spreadsheetml/2006/main' => 's'      
},
keep_original_prefix => 1,
pretty_print => 'indented',
twig_handlers => {'/s:workbook/s:sheets' => sub{Add_Sheet_1(@_)}}
);
$tw1->parsefile($fileName1);
open($fh1, '>:encoding(UTF-8)', $fh1_filename);
$tw1->flush($fh1);
close $fh1;
}   
#
#
#
sub Add_Sheet_1{
    my ( $twig, $tag) = @_;
    my $var1='x:sheet';
    $tag->insert_new_elt( 'last_child', => $var1=>{name=>'Sheet_N1',sheetId=>'200'});
}

For the case where the prefix is "x:" this code generates the desired result.

However, in general, the prefix is unknown. My handler maps the unknown prefix to the prefix "s:" So finding the correct element poses no problem.

However, I cannot find any way of adding a new child element with the correct prefix.

Is there any way I can get the original prefix from the file and use it when adding my new child element?