It's line 2454 in CGI.pm (v3.39):

    } elsif ($default) {
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which makes $default values of zero (=false) be ignored. ($default is where your $CONFIG{convert_links_to_friendly} value ends up in.)

The fix would apparently be

    } elsif (defined $default) {
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but I can't tell for sure whether that would maybe break something else (don't think so), or what the motivation for not using defined might have been...