As your Data::Dumper output shows, there is significant whitespace after "bandwidth". One way to get it to work (and eliminate the warning) is to change:

my $bandwidth =  $xml->{exec_command}->{xml_show_result}->{xml_show_re
+source_usage}->{ru_entry}->{bandwidth}->{ru_current};
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to:

my $bandwidth =  $xml->{exec_command}->{xml_show_result}->{xml_show_re
+source_usage}->{ru_entry}->{'bandwidth           '}->{ru_current};
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Update: But a better way (after searching the POD for "space") is to use NormaliseSpace:

my $xml = XMLin($tmpfile, ForceArray => 0, SuppressEmpty => 1, KeyAttr
+ => 'ru_resource', NormaliseSpace => 1 );
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