In addition to what's been said, it makes no sense to use an xpc in one place and not the other.

my $xc = XML::LibXML::XPathContext->new($root);
$xc->findnodes('/HOSTNAME/PATROL/ACL/USERNAME')
$sections->findnodes('./PERMISSION')
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should be

$root->findnodes('/HOSTNAME/PATROL/ACL/USERNAME')
$sections->findnodes('./PERMISSION')
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or

my $xc = XML::LibXML::XPathContext->new( $root );
$xc->findnodes('/HOSTNAME/PATROL/ACL/USERNAME')
$xc->findnodes('./PERMISSION', $sections)
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An xpc isn't needed here since your XML doesn't use namespaces, but there's no harm in using one. Just be consistent about it.

Note that the leading to "./" is useless (whether an xpc is used or not). The following will do:

$xc->findnodes('PERMISSION', $section)
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