foreach my $val (keys ${\( STUFF() ) } ){ ... }

Here, keys is iterating over a hash reference. Quoth the docs (from 5.14):
    Starting with Perl 5.14, "keys" can take a scalar EXPR, which
    must contain a reference to an unblessed hash or array. The
    argument will be dereferenced automatically. This aspect of
    "keys" is considered highly experimental. The exact behaviour
    may change in a future version of Perl.
And indeed, this feature was tried, found wanting, and finally cast into Outer Darkness with Perl version 5.24. (And similarly for values, each, push, pop and, I think, some others.) See also Postfix Dereference Syntax and circumfix dereference syntax, discussed therein.

c:\@Work\Perl\monks>perl -wMstrict -le
"print qq{perl version: $] \n};
 ;;
 use constant {
   STUFF => {
     'bizz' => 'foe',
     'bazz' => 'fie',
     'bozz' => 'fee',
     },
   };
 print ${\( STUFF() ) };
 print STUFF;
 ;;
 foreach my $val (keys ${\( STUFF() ) } ){
   my $subroutine = ${\( STUFF() ) }->{ $val };
   print qq{A: '$val' -> '$subroutine'};
   }
 ;;
 for my $k (keys %{ STUFF() }) {
   my $value = STUFF->{$k};
   print qq{B: '$k' -> '$value'};
   }
"
perl version: 5.014004

HASH(0x4340ac)
HASH(0x4340ac)
A: 'bozz' -> 'fee'
A: 'bazz' -> 'fie'
A: 'bizz' -> 'foe'
B: 'bozz' -> 'fee'
B: 'bazz' -> 'fie'
B: 'bizz' -> 'foe'
[download]

(The circumfix dereference syntax is more widely supported — and I can't give an example of postfix dereference syntax anyway!)