$s = '1234567890'x10;;

cmpthese -1, {
    a=>q[ my $x  = substr $s, $_, 10 for 0 .. 99],
    b=>q[ my($x) = m[^.{$_}(.{10})]  for 0 .. 99] 
};;
     Rate     b     a
b  1113/s    --  -97%
a 42708/s 3738%    --