use v5.10;

my $input;

local $" = ", ";

my $parser = qr{
    (?{
         say "before:\n\@- = (@-)\t\t ".scalar(@-)." items\n\@+ = (@+)\t ".scalar(@+)." items\n";
    })
    ^
    ((?&expr))
    ((?&expr))
    \z
    (?{
         say "after:\n\@- = (@-)\t\t ".scalar(@-)." items\n\@+ = (@+)\t ".scalar(@+)." items\n";
    })

    (?(DEFINE)
        (?<expr>
            (.) (.)
            (?{
                 say "expr:\n\@- = (@-)\t ".scalar(@-)." items\n\@+ = (@+)\t ".scalar(@+)." items\n";
            })
        )
    )
}x;

$input = "abcd";
chomp($input);
if ($input =~ $parser) {
  say "matches: ($&)";
  say "At the very end:\n\@- = (@-)\t ".scalar(@-)." items\n\@+ = (@+)\t ".scalar(@+)." items\n";
}
__END__

##</code><code>##

Compiling REx "%n    (?{%n         say %"before:\n\@- = (@-)\t\t %".scalar("...
synthetic stclass "ANYOF[\0-\11\13-\377][{unicode_all}]".
Final program:
   1: EVAL (3)
   3: BOL (4)
   4: OPEN1 (6)
   6:   GOSUB3[+19] (9)
   9: CLOSE1 (11)
  11: OPEN2 (13)
  13:   GOSUB3[+12] (16)
  16: CLOSE2 (18)
  18: EOS (19)
  19: EVAL (21)
  21: DEFINEP (23)
  23: IFTHEN (44)
  25:   OPEN3 'expr' (27)
  27:     OPEN4 (29)
  29:       REG_ANY (30)
  30:     CLOSE4 (32)
  32:     OPEN5 (34)
  34:       REG_ANY (35)
  35:     CLOSE5 (37)
  37:     EVAL (39)
  39:   CLOSE3 'expr' (44)
  41:   LONGJMP (43)
  43:   TAIL (44)
  44: END (0)
floating ""$ at 2..2147483647 (checking floating) stclass ANYOF[\0-\11\13-\377][{unicode_all}] minlen 2 with eval 

##</code><code>##

before:
@- = (0)		 1 items
@+ = (0, , , , , )	 6 items

expr:
@- = (0, , , , 0, 1)	 6 items
@+ = (2, , , , 1, 2)	 6 items

expr:
@- = (0, 0, , , 2, 3)	 6 items
@+ = (4, 2, , , 3, 4)	 6 items

after:
@- = (0, 0, 2)		 3 items
@+ = (4, 2, 4, , , )	 6 items

matches: (abcd)
At the very end:
@- = (0, 0, 2)	 3 items
@+ = (4, 2, 4, , , )	 6 items