... each ... newer Perl ...

Perl version 5.12+, and 5.14+ has the "highly experimental" feature of allowing each to directly take a reference:

>perl -wMstrict -le
"my %matrix = (NI => [2, 1, 1, 3, 4] );
 ;;
 my @NI_index;
 my $min;
 while (my ($i, $v) = each $matrix{NI}) {
   if(not defined $min or $v < $min) {
     $min = $v;
     @NI_index = ();
     }
   push @NI_index, $i if $v == $min;
   }
 print qq{@NI_index};
"
1 2
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