$ perl -del
  DB<2> x 0+( () = join('',('<c>','x'x((1<<30)-10),'</c>')x2) =~ m{<c.*?/c>}g )
0  2
  DB<1> x 0+( () = join('',('<c>','x'x((1<<30)-10),'</c>')x3) =~ m{<c.*?/c>}g )
0  0