use strict;
use warnings;
use List::Util qw/min/;

my %matrix = ( NI => [ 1, 1, 3, 4 ] );
my @NI_index = grep { $matrix{NI}->[$_] == min @{ $matrix{NI} } } 0 .. $#{ $matrix{NI} };
print "@NI_index";

##</code><code>##

0 1