According to the usage examples for Lingua::EN::Tagger,

    # Get a list of all nouns and noun phrases with occurrence counts
    my %word_list = $p->get_words( $text );
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Assuming the comment there is correct, it appears that Lingua::EN::Tagger itself does what you're asking for. (If not, I would think it should be simple enough to loop over its output and build the frequency count yourself.)