Any ideas why it only returns the plain text portion, and no html?

Sure ;) Look at this, here are four returns and first always works:

#this one returns you text/plain part
    return $ct{'text/plain'} if exists $ct{'text/plain'};
# and these are not executed it there's text/plain part
    return $ct{'text/html'} if exists $ct{'text/html'};
# these are not executed at all an they are not correct!
# parts[0] may contain text/html and parts[1] text/plain
    return $parts[0] if $which =~ /text/; 
    return $parts[1] if $which =~ /html/
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