my @state;
while (<>) {
   push @state, 'input'    if /<input>/;
   push @state, 'ordered'  if /<ordered-list>/;
   push @state, 'bulleted' if /<bulleted-list>/;
   pop @state              if /<end-list>/ || /<\/input>/;

   my $state = $state[$#state];
   s/<item>/<ordered-item>/g  if $state eq 'ordered';
   s/<item>/<bulleted-item>/g if $state eq 'bulleted';

   print;
}