Hi

Hmm splain has heard of the error (its in perldiag)

$ cat 2
Can't coerce UNKNOWN to string in substitution iterator at ./coerce.pl
+ line 11, <> line 12.

$ splain 2
Can't coerce UNKNOWN to string in substitution iterator at ./coerce.pl
+ line 11,
        <> line 12 (#1)
    (F) Certain types of SVs, in particular real symbol table entries
    (typeglobs), can't be forced to stop being what they are.  So you 
+can't
    say things like:

        *foo += 1;

    You CAN say

        $foo = *foo;
        $foo += 1;

    but then $foo no longer contains a glob.
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I can't replicate the error in v5.16.1 but I can in v5.18.2

The output starts to differ at line 59 , so I modify the input to just the part that makes error

5
1 3 8 14 19 26 33
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Adding use re 'debug'; makes it run to completion , but output is not identical

Now looking at the regex, and checking perlre,

I see you're using (??{}) , but the return value (a regex pattern) is either X or nothing ... and there is no X in the input,

so I switch to using (?{}) where return value isn't a regex pattern, and with this change both versions of perl produce the same output

Now its not the same output as before with (??{}) but I dont think thats important