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use XML::LibXML;

my $dom = 'XML::LibXML'->load_xml(location => 'file.xml');
my $digits = $dom->findvalue('//stlib:mem_bit');
print $digits, "\n";

<r xmlns:stlib="http://stlib.xml.ns">
  <ch>
    <stlib:mem_bit>72</stlib:mem_bit>
  </ch>
</r>

#!/usr/bin/perl
use warnings;
...
$xpc->registerNs(s => 'http://stlib.xml.ns');
my $digits = $xpc->findvalue('//s:mem_bit', $dom);
print $digits, "\n"