note pilcrow <tt>perlfunc</tt>'s entry for <tt>undef</tt> suggests this behavior on slices, if one squints: <blockquote> Undefines the value of EXPR, which must be an lvalue... Saying undef $hash{$key} will probably not do what you expect... </blockquote> 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: <code> $ perl -le 'undef @h{qw|a b c|}; print "$]: ", join " ", keys %h' 5.00404: a b c </code> 280658 435223