The  use re 'debug'; statement/pragma (see re) is useful to gain insight into what the Perl regex compiler thinks you wrote.

The debug output is just a teensy bit opaque, but some insight can be had just by noting that in both cases below, the  'a' literal substring part of the regex is denoted as
     6:   EXACT <a> (8)
whereas in the case that doesn't work, what you think is a counted quantifier is denoted
    14:   EXACT <{0, 5}> (17)
(i.e., the regex compiler thinks it's a  '{0, 5}' literal substring), versus
    13:   CURLY {0,5} (16)
    15:     REG_ANY (0)
in the case of the correctly written counted quantifier (the one that works as expected).

>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 m
+inlen 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})"
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The YAPE::Regex::Explain module can also offer helpful insight, but unfortunately it doesn't support regex constructs much beyond Perl version 5.6 and that's just what you're using!