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perlfunc's entry for undef suggests this behavior on slices, if one squints:
Undefines the value of EXPR, which must be an lvalue... Saying undef $hash{$key} will probably not do what you expect...
That is, taking "undef EXPR" as (usually) equivalent to "EXPR = undef", the behavior is sensible, even expected. At any rate, a warning seems inappropriate, as this feature is long-standing:
$ perl -le 'undef @h{qw|a b c|}; print "$]: ", join " ", keys %h' 5.00404: a b c

In reply to Re^4: Perl Idioms Explained - keys %{{map{$_=>1}@list}} by pilcrow
in thread Perl Idioms Explained - keys %{{map{$_=>1}@list}} by broquaint

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