use strict;
use warnings;
use Data::Dumper;

my @data = (
	"o1	n1	a1\n",
	"o2	n2	a2\n",
	"o3	n1	a3\n",
);

my %caseCount;
for my $caseMetrics (@data) {
	my $caseNumber = (split /\t/, $caseMetrics)[1];
	$caseCount{$caseNumber}++;
}

print Dumper(\%caseCount);

__END__

 Output...

$VAR1 = {
          'n2' => 1,
          'n1' => 2
        };