G'day lightoverhead,

I ran that code and got the same result as you:

$ perl -Mstrict -Mwarnings -E '
    "abcdef" =~ / .+ (?{say "Matched so far: $&"}) bcdef $/x;
'
Matched so far: a
[download]

I then stepped through the code using Regexp::Debugger:

$ perl -Mstrict -Mwarnings -E '
    use Regexp::Debugger;
    "abcdef" =~ / .+ (?{say "Matched so far: $&"}) bcdef $/x;
'
[download]

This showed each of the six "Matched so far: ..." lines in the same order as you have them. It also showed backtracking behaviour as I, and presumably you, would expect.

The problem seems to lie with just the output; however, I don't know what that problem is.