Again, you are not checking for errors. The documentation for getPageContentTree says (bold by me):

Retrieves a parsed page content data structure, or undef if there is a syntax error or if the page does not exist.

So, check for errors, and you should find the errors:

my $content = $pdf->getPageContentTree($page);
if (! $content) {
    warn "Couldn't get content for page $page: $CAM::PDF::errstr";
    # thanks to Anonymous Monk for pointing out the error variable
} else {
    # process the page
};
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