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The former interprets the slice as a scalar list, and undefs the last element only. The reason all the keys get defined is that it puts the slice in lvalue context, and autovivification occurs. The latter is equivalent to assigning a list to a list.
I think I see your point. Would you then have expected, in nature, the following to be equivalent:
$h{c} = undef; # 1 undef $h{c}; # 2 undef @h{qw|a b c|}; # 3
with no. 3 taking the hash slice as a plain list, and that in a scalar context?

In reply to Re^6: Perl Idioms Explained - keys %{{map{$_=>1}@list}} by pilcrow
in thread Perl Idioms Explained - keys %{{map{$_=>1}@list}} by broquaint

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    [Corion]: Aaah - you should be able to do this with overload, but I would hit somebody really hard if they constructed objects that are true but the empty string, and you not knowing about the domain knowledge where this makes sense
    [Eily]: you could tie a variable into not having the same value each time, if you like to make people who try to debug your code facepalm
    [Corion]: perl -wle 'package o; use overload q("") => sub {warn "str"; ""}, bool => sub{warn "bool"; 1}; package main; my $o={}; bless $o => o; print "Yay" if ($o && !length($o))'
    [Corion]: But people writing such code should document the objects they construct and why it makes sense for an object to be invisible as string while being true in a boolean context
    [hippo]: That's equal parts clever and horrendous.
    [Eily]: the overload version wouldn't return true with "$x" && !length $x though, I guess
    [hippo]: The more I look at this code, the more $x is a plain old scalar and the more this condition will never be true. I'm calling it a bug at this point.
    [hippo]: Thanks for your input which has soothed my sanity (a little)
    [Corion]: Eily: Sure - if you force both things into stringy things, then you break that magic. But that would also mean that you changed the expression, as now $x = 0.00 will be true instead of false as it were before
    [Corion]: Ah no, at least in my feeble experiments that doesn't change the meaning

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