I don't see anything before my use of getElementByClassName that would have put us in the context of that subclass

I do ;-) In your first example you have:

my $html = HTML::TagParser->new($line);
my @elem = $html->getElementsByClassName($class);
[download]

@elem now contains a list of HTML::TagParser::Element objects
In your second example you have:

my $html = HTML::TagParser->new($line);
my $body = $html->getElementById($id);
my @elem = $body->getElementsByClassName($class);
[download]

Here $body is a single HTML::TagParser::Element object, and you can't call getElementsByClassName on that object. Try this (untested):

my $html = HTML::TagParser->new($line);
my $body = $html->getElementById($id);
$body = $body->subTree(); # $body is now a new HTML::TagParser object
my @elem = $body->getElementsByClassName($class);
[download]

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