Re: Returning indices with same values for array within hash.

[AnomalousMonk]

Another variation, also on the shoulders of others:
Using an intermediate 'convenience' variable
    my $array_ref = $matrix{NI};
to hold the reference of interest can greatly simplify/clarify matters. It doesn't buy you much in this particular case, but can be very helpful with complex, deeply nested data structures.

>perl -wMstrict -le
"use List::Util      qw(min);
 use List::MoreUtils qw(indexes);
 ;;
 my %matrix  = ( NI => [ 1, 5, 1, 3, 4 ] );
 ;;
 my @indices = do {
   my $array_ref = $matrix{NI};
   my $mat_min = min @$array_ref;
   indexes { $_ == $mat_min } @$array_ref;
   };
 print qq{(@indices)};
"
(0 2)

Update: And, of course, the guts of a  do { ... } block can usually be easily lifted out and plunked down in a subroutine:

>perl -wMstrict -le
"use List::Util      qw(min);
 use List::MoreUtils qw(indexes);
 ;;
 my %matrix  = (
   foo => { bar => [ 'oops', { NI => [ 1, 5, 1, 3, 4 ], }, ], },
   );
 ;;
 my @indices = get_indices($matrix{foo}{bar}[1]{NI});
 print qq{(@indices)};
 ;;
 sub get_indices {
   my ($ar) = @_;
   ;;
   my $mat_min = min @$ar;
   return indexes { $_ == $mat_min } @$ar;
   }
"
(0 2)