perl -MXML::Parser -le '$p=XML::Parser->new(
 Handlers=>{Start=>sub{$k.=".$_[1]";$h{$k}++},
             End=>sub{$k=~s/.$_[1]$//;}});
 $p->parsefile(shift);
 END{print "$h{$_}\t$_" for (sort keys %h)}'   some_file.xml

(Line-breaks added for legibility; the first time I did this, it really was all on one line -- honest!)