> I don't think exists(&subname) should "autovivify" anything.

Complex things are happening here, and I don't know in which order to untangle them

• a sub &name lives in a slot of the stash entry *name which is accessible via $main::{name} or $::{name} for short
• so if a sub is created, so is the glob autovivified
• exists might autovivify entries on higher level of the stash tree
• but this autovivification seems to already happen at compile time when the symbol &non is seen, even inside exists! Please note how $::{other} doesn't exist
• your use of no autovivification is changing what exists &sub returns, it's an empty list () instead of an empty string "" (both are false)

Compare my code experiments.

use strict;
use warnings;
use feature 'say';
no autovivification;
use Data::Dump qw/pp/;
my @ary;
pp exists $::{non};
pp exists $::{other};
pp exists (&non) ;

push @ary,  exists (&non) ;
say scalar(@ary)
[download]

1     # *non exists after compilation of &non
""       # *other doesn't exist
()       # sub &non doesn't exist but empty list returned
0        # @ary has consequently no elements
[download]

Edit

After commenting out #no autovivification

 1
""
""
1
[download]

Update

My guess is that there is actually always an autovivification happening at compile time creating the glob, which has side effects on exists at run time.

I'd say this implementation is buggy!

If you need a workaround, I'd try to first checking if the glob exists before checking the slot and avoiding exists &name

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