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

##</code><code>##

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