I think it is a just variation on the "will not stay shared" theme. If you insert a named subroutine that refers to @o and use warnings; use strict; it will give you a warning: 

use warnings;
use strict;

sub foo {
    my @o;
    sub demonstration { @o };
}

[download]

Variable "@o" will not stay shared at xxx.pl line 6.

Changing "use warnings" to "use diagnostics" prints the postcard version of the message:

Variable "@o" will not stay shared at xxx.pl line 6 (#1) (W closure) An inner (nested) named subroutine is referencing a lexical variable defined in an outer subroutine.

When the inner subroutine is called, it will probably see the value of the outer subroutine's variable as it was before and during the *first* call to the outer subroutine; in this case, after the first call to the outer subroutine is complete, the inner and outer subroutines will no longer share a common value for the variable. In other words, the variable will no longer be shared.

Furthermore, if the outer subroutine is anonymous and references a lexical variable outside itself, then the outer and inner subroutines will never share the given variable.

This problem can usually be solved by making the inner subroutine anonymous, using the sub {} syntax. When inner anonymous subs that reference variables in outer subroutines are called or referenced, they are automatically rebound to the current values of such variables.

It's unfortunate that it doesn't seem to warn about that when the variable is used in (?{code}).

In reply to Re: Multiple uses of (?{ code }) do not appear to be called by quester
in thread Multiple uses of (?{ code }) do not appear to be called by bsdz

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