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})"
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!
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