Or is there another way to do it?

More of a variation on a theme (which builds on the previous answers):

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";
[download]

Output:

 1:57 >perl 774_SoPW.pl
0 2
 1:57 >
[download]

See List::MoreUtils.

Hope that helps,

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