mention that $1 etc. is reset even without any /()/'s

[Dan Jacobson]

perlop and perlretut seem not to mention that $1, $2 ... are reset
even when one doesn't use ()'s.

"Hello World" =~ /W(o)rld/;
print $1; #prints "o"
"Hello World" =~ /World/;
print $1; #uninitialized value!