I don't think you are missing something, it looks like it really is a bug (or at least an oversight). The code that generates this html looks like this (this is the end of a sub generated by _make_tag_func which is called by AUTOLOAD in CGI.pm):

return $XHTML?"\L<br\E$attr />":"\L<br\E$attr>" unless @_;
my($tag,$untag) = ("\L<br\E$attr>","\L</br>\E");
my @result = map { "$tag$_$untag" } 
    (ref($rest[0]) eq 'ARRAY') ? @{$rest[0]} : "@rest";
return "@result";
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It seems that the intention of this is generate a container-style tag if given arguments, and a standalone tag without arguments. The oversight is that if you call it as an object method it doesn't deal with the fact that the object is at $_[0]. So you end up with this effect:

#!/usr/bin/perl -w
##################
use CGI qw/:standard/;

print "foo".br()."\n";

my $q = new CGI;
print "foo".$q->br()."\n";
__END__
foo<br />
Use of uninitialized value in join or string at (eval 2) line 15.
foo<br></br>
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That uninitialized value warning comes from the generated sub above. I think the moral is that nobody ever expected the html generating functions to be called as object methods, and it should work fine if you import them with :html and call them as regular subs.