how does ... differ in execution from ...

Hm. "in execution"?

1. The result -- the count -- is the same.
2. The runtime is less than half:

   $s = 'ab'x 10e6;;
   
   cmpthese -1, {
       a => q[ 
           my $n = 0; 
           while( $s =~ m[ab]g ){
               $n++
           };
       ],
       b=>q[
           my $n = 0; 
           ++$n while $s =~ m[ab]g;
       ],
   };;
   
     s/iter    a    b
   a   2.31   -- -37%
   b   1.45  59%   --
   [download]

3. It is easier to read.

Don't use post increment if you aren't using the pre-increment value. Don't create scopes you don't need.

Or, get into the habit of not doing anything you don't need to.