Actually, you don't have to put it in an attribute - it works just as well from a child element as long as you haven't used forcearray on that element name. I'd recommend calling XMLin something like this:

my $config = XMLin($filename, 
  keyattr    => {picture => 'Name'}, 
  forcearray => ['picture']
);
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Then you could access the required value like this:

print $config->{pictures}->{picture}->{'test.jpg'}->{Mtime}, "\n";
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