Since you didn't show any input data or expected output, it is difficult to understand what you are trying to accomplish. So, I'll just create some simple input with 3 lines, 3 tab-separated columns per line. This will count the number of times each string appears in the 2nd column:

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
        };
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Since you only seem to use 1 of the columns, just keep the one you want when you split. Splitting into multiple variables is hardly ever a good approach anyway; typically, you would split into an array.