>perl -wMstrict -le
"use re 'debug';
 ;;
 my $rx = qr/(?<WORD>\b\w*a\b)(?<EXTRA>.{0, 5})/;
"
Compiling REx "(?<WORD>\b\w*a\b)(?<EXTRA>.{0, 5})"
Final program:
   1: OPEN1 'WORD' (3)
   3:   BOUND (4)
   4:   STAR (6)
   5:     ALNUM (0)
   6:   EXACT <a> (8)
   8:   BOUND (9)
   9: CLOSE1 'WORD' (11)
  11: OPEN2 'EXTRA' (13)
  13:   REG_ANY (14)
  14:   EXACT <{0, 5}> (17)
  17: CLOSE2 'EXTRA' (19)
  19: END (0)
floating "{0, 5}" at 2..2147483647 (checking floating) stclass BOUND minlen 8
Freeing REx: "(?<WORD>\b\w*a\b)(?<EXTRA>.{0, 5})"

>perl -wMstrict -le
"use re 'debug';
 ;;
 my $rx = qr/(?<WORD>\b\w*a\b)(?<EXTRA>.{0,5})/;
"
Compiling REx "(?<WORD>\b\w*a\b)(?<EXTRA>.{0,5})"
Final program:
   1: OPEN1 'WORD' (3)
   3:   BOUND (4)
   4:   STAR (6)
   5:     ALNUM (0)
   6:   EXACT <a> (8)
   8:   BOUND (9)
   9: CLOSE1 'WORD' (11)
  11: OPEN2 'EXTRA' (13)
  13:   CURLY {0,5} (16)
  15:     REG_ANY (0)
  16: CLOSE2 'EXTRA' (18)
  18: END (0)
floating "a" at 0..2147483647 (checking floating) stclass BOUND minlen 1
Freeing REx: "(?<WORD>\b\w*a\b)(?<EXTRA>.{0,5})"