The output of the following line of code is different than what is described in the text:
"abcdef" =~ / .+ (?{say "Matched so far: $&"}) bcdef $/x;

The text states that this prints the following:

Matched so far: abcdef
Matched so far: abcde
Matched so far: abcd
Matched so far: abc
Matched so far: ab
Matched so far: a

But I am finding that only the last line gets printed:

Matched so far: a

One solution to get the output that the text describes is to slightly modify the code example to the following:

"abcdef" =~ / .+ (?{say "Matched so far: $&"}) .cdef $/x;