Why doesn't Perl have a warning for the [@hash{@array} = 'foo'] case?

I would say because an empty hash slice assignment | index list is a perfectly valid thing. It's equivalent to
    $hash{$_} = 'foo' for @array;
which also does not (and should not) warn.

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my %hash = qw(a 1  b 2);
 my @array;
 @hash{@array} = 'foo';
 dd \%hash;
 ;;
 $hash{$_} = 'foo' for @array;
 dd \%hash;
"
{ a => 1, b => 2 }

{ a => 1, b => 2 }
[download]