I can confirm the behaviour in 5.18.2 and 5.20.1, but running the code in blead (720c8c4f57a8be0960c9068d899e9d111e143382) finishes the code without error.

Check also splain for the message: 

Can't coerce UNKNOWN to string in substitution iterator at - 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.

[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

[download]

In reply to Re: Can't coerce UNKNOWN to string in substitution iterator by choroba
in thread Can't coerce UNKNOWN to string in substitution iterator by rsFalse

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