Are "$hash{$_} ||= 1 + keys %hash" and variants well defined or not?

[demerphq]

Should the following two lines of code do the same thing?

$hash{$_} ||= 1 + keys %hash for @list;
$hash{$_} = $hash{$_} || 1 + keys %hash for @list;

IMO the second one should be well defined. tye and diotalevi think that both are undefined, I could be conviced about the first, but I need more convincing on the second.

This came up because I was being cheeky in Re^3: Perl one-liner to remove duplicate entries from PATH and wound up scratching my head.

use Data::Dump::Streamer;
my (%hash,%mash);
@list=('A'..'C');
$hash{$_} ||= 1 + keys %hash for @list;
$mash{$_} = $mash{$_} || 1 + keys %mash for @list;
Dump(\%hash,\%mash)->Names('*hash','*mash')->Out();
__END__
%hash = (
          A => 2,
          B => 3,
          C => 4
        );
%mash = (
          A => 1,
          B => 2,
          C => 3
        );