use strict;
use warnings;
use List::Util      qw( min );
use List::MoreUtils qw( indexes );

my %matrix  = ( NI => [1, 5, 1, 3, 4] );
my $minimum = min @{ $matrix{NI} };
my @indices = indexes { $_ == $minimum } @{ $matrix{NI} };
print "@indices";