Your XML document has a XML namespace defined. Use XML::LibXML::XPathContext to handle namespaces in XML::LibXML.

Also, the correct XPath expression to find tmmTableFilenameMap anywhere in the document is //tmmTableFilenameMap, otherwise the searching only happens at the current node.

#! /usr/bin/perl
use strict;
use warnings;

use XML::LibXML;

my $filename = shift;

my $xpc = 'XML::LibXML::XPathContext'->new;
$xpc->registerNs(m => 'urn:mavenir:ns:yang:wrg-sc:6.0.2.8');

my $dom = 'XML::LibXML'->load_xml(location => $filename);

for my $delete ($xpc->findnodes('//m:tmmTableFilenameMap', $dom)) {
    my ($parent) = $delete->findnodes('..');
    $parent->removeChild($delete);
}

print $dom->serialize;
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It gets even shorter using XML::XSH2:

open webrtcc_system_static_config_data_platremoved.xml ;
register-namespace m urn:mavenir:ns:yang:wrg-sc:6.0.2.8 ;
delete //m:tmmTableFilenameMap ; 
save :b ;
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