it is a syntax error.

Depends on your version of Perl. It's called a index-value slice. It returns key,val,key,val,...

Really? In which release of perl did that work then?:

C:\test>\perl5.18\bin\perl -E"my %h=1..10; my $r = \%h; print %{ $r }[
+ 1,2 ];"
syntax error at -e line 1, near "}["
Execution of -e aborted due to compilation errors.

C:\test>\perl5.18\bin\perl -E"my @a=1..10; my $r = \@a; print %{ $r }[
+ 1,2 ];"
syntax error at -e line 1, near "}["
Execution of -e aborted due to compilation errors.
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Or is this a preview of Yet Another Mis-Feature that'll have to be deprecated in a year or so?

It will be sad indeed to see Perl die as a result of self-inflicted injuries.