If you insert

warn ${$stash}{$symbol},"\n";
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before line 26, you see what happens. Version 5.20.2 displays

*main::foo
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whereas v5.26.0 displays something like

CODE(0x123abc)
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Sigh. The glob dereferencing to get at the code reference of a glob must have changed somewhere after 5.20.2, so I'll have to rewrite the symbol handling part. I'll update the OP as soon as this is done.

update: OP updated.