$ perl -MDDS -w -le " my $hr; $hr->{stuff}; Dump($hr)"
$HASH1 = {};

$ perl -MDDS -Mwarnings -le " my $hr; $hr->{stuff}; Dump($hr)"
Useless use of hash element in void context at -e line 1.
$HASH1 = {};
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Curiously, my little test reveals Data::Dump::Streamer breaks the -w switch .

I see what you mean.  So, one could conclude that getting no warning in the OP's case (with map) is in fact the appropriate behavior (as the $hr->{$_} is not useless), while getting the warning with for seems more questionable  (in which case I wouldn't consider it a bug to silence a useless "Useless use of..." warning :)

One could if you ignore intent, but I wouldn't support that conclusion. The warning deals with return value and where it ends up. perldiag says:

... Very often this points not to stupidity on your part, but a failure of Perl to parse your program the way you thought it would...

so if you were to adapt perldiag example with andreas' example you get

#~ perl -MDDS -Mstrict -Mwarnings -le " my $hr; map { @{$hr->{$_}}{qw'
+ a b c '} = $_,$_,$_ ; } 1; Dump($hr) "
#~ perl -MDDS -Mstrict -Mwarnings -le " my $hr; map { @{$hr->{$_}}{qw'
+ a b c '} = ( $_,$_,$_ ); } 1; Dump($hr) "
#~ perl -MDDS -Mstrict -Mwarnings -le " my $hr; $_ = 1; @{$hr->{$_}}{q
+w' a b c '} = $_,$_,$_ ; Dump($hr) "
#~ perl -MDDS -Mstrict -Mwarnings -le " my $hr; $_ = 1; @{$hr->{$_}}{q
+w' a b c '} = ( $_,$_,$_ ); Dump($hr) "


$ perl -MDDS -Mstrict -Mwarnings -le " my $hr; map { @{$hr->{$_}}{qw' 
+a b c '} = $_,$_,$_ ; } 1; Dump($hr) "
$HASH1 = { 1 => {
           a => 1,
           b => undef,
           c => undef
         } };

$ perl -MDDS -Mstrict -Mwarnings -le " my $hr; map { @{$hr->{$_}}{qw' 
+a b c '} = ( $_,$_,$_ ); } 1; Dump($hr) "
$HASH1 = { 1 => {
           a => 1,
           b => 1,
           c => 1
         } };

$ perl -MDDS -Mstrict -Mwarnings -le " my $hr; $_ = 1; @{$hr->{$_}}{qw
+' a b c '} = $_,$_,$_ ; Dump($hr) "
Useless use of a variable in void context at -e line 1.
Useless use of a variable in void context at -e line 1.
$HASH1 = { 1 => {
           a => 1,
           b => undef,
           c => undef
         } };

$ perl -MDDS -Mstrict -Mwarnings -le " my $hr; $_ = 1; @{$hr->{$_}}{qw
+' a b c '} = ( $_,$_,$_ ); Dump($hr) "
$HASH1 = { 1 => {
           a => 1,
           b => 1,
           c => 1
         } };

$
$
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