@a = map[ map int(rand$_), 1 .. 10],1..10;;
cmpthese -1,{
    a=>q[ map $_ /= 1, @$_ for @a; ],
    b=>q[ for my $r( @a ){ $_ /= 1 for @$r } ], 
};;
     Rate    b    a
b 52726/s   -- -25%
a 70388/s  33%   --