But I am not explicitly telling Perl to autovivify, rather I am (intending to) just reference the values, in this case using a slice.

Refer to my simpler example above. Using see_args( @foo{qw(BAR BAZ)} ); causes result:

at start: BAR '<absent>'
at end: BAR '<undef>'
[download]

But changing that to see_args( $foo{BAR}, $foo{BAZ} ); results in:

at start: BAR '<absent>'
at end: BAR '<absent>'
[download]

This is most perplexing to me.