use strict;
use warnings;

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;          # found new minimum
        @NI_index = ();     # empty list
    }
    push @NI_index, $i if $v == $min;
}
print "@NI_index\n";