If I'm understanding you correctly, perhaps the following will be helpful:

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

my ( %hashTable, %groupedHash );

while (<DATA>) {
    my ( $key, $val ) = split;
    $hashTable{$key} = $val;
}

groupByCol2( \%hashTable, \%groupedHash );

print Dumper \%groupedHash;

sub groupByCol2 {
    my ( $hashTableRef, $groupedHashRef ) = @_;
    for my $key ( keys %$hashTableRef ) {
        push @{ $$groupedHashRef{ $$hashTableRef{$key} } }, $key;
    }
}

__DATA__
name1    group1
name2    group2
name3    group1
name4    group4
name5    group2
name6    group7
name7    group4
name8    group5
[download]

Output:

$VAR1 = {
          'group4' => [
                        'name7',
                        'name4'
                      ],
          'group2' => [
                        'name2',
                        'name5'
                      ],
          'group7' => [
                        'name6'
                      ],
          'group1' => [
                        'name1',
                        'name3'
                      ],
          'group5' => [
                        'name8'
                      ]
        };
[download]

Send two hash references to the subroutine groupByCol2--the first a reference to your 'hash table' with repeating values and the second to a hash for a hash of arrays (HoA). The second has holds the 'grouped' names.

Thanks Kenosis. I've started working on my logic. I tried with you code, but it didnt parse keys and value correctly. Insted got output like:

Output:

is mixed with column1 and column2 data.

$VAR1 = {
          'group4' => [
                        'group2','name7',
                        'name4','group7',
                      ],
          'group5' => [
                        'name8'
                      ]
        };
[download]

Then, somehow, entires from column 2 (the groups' column) became keys in your 'hash table.' You need to discover how this happened, since the groupByCol2 subroutine merely uses your 'hash table's' values as keys and associates them with references to anonymous arrays which contain name entires.

I can't be bothered to read your node because you couldn't be bothered to format it nicely, but in answer to your title: use YAML.

Well, stressed of coding with google search, understanding working of perl language is fun.

I've decided to use hash table and parsing %$hash_ref into while loop search for column2 repetitive values. Next use for loop to create a loop until every repetitive value is parsed and concatenated to existing outfile. Next, start iteration to find other repetitive word from column2 (values).

Good part is Learning Perl with Monks feedback makes Perl easy.

Moving forward, I tried to initially just print hash table using hashref. Please help me try to understand. I know somehow I'm referring to array address, instead of actual value stored in memory.

Expected Output:

C_WREN => Group1
CAL_CLK => Group1
RX_IBIAS_2_25U[4:0] => Group3
REF_RATE[9:0] => Group3
AVD0 => Group4
SIDDQ => Group7
[download]

Main Program

sub mainCSV 
{
    # Open the CSV input file
    open (my $infile_CSV1, '<', "$infile_CSV") or die "Unable to open 
+$infile_CSV: $!\n";
    my %hash  = (); 
    my $hash_ref = \%hash;
    
    while (my $line = <$infile_CSV1>) 
    {
        chomp;
        $line =~ s/\s*\z//;
        my @array_CSV = split /,/, $line;
        my $key_CSV = shift @array_CSV;
        if ($hash{$key_CSV})
        {
            warn "Duplicate key '$key_CSV'";
        };
        $hash{$key_CSV} = \@array_CSV;
    }
    
    # Explicit scalar context 
    my $size = scalar keys %hash; 

    # Prints Number of Pins (Hash size...)
    print "Number of Pins: $size\n"; 
    
    # Open the output file and save hash in $outfile_RX_CSV
    open (my $outfile2, '>', "$outfile_CSV") or die "Unable to open $o
+utfile_CSV: $!\n";
    print $outfile2 Dumper(\%hash); 
    close $outfile2;
    print "Stored $size list of pins in $outfile_CSV file.\n";
    
    
    # Method 1 to print hashref table
    while( my ($key, $value) = each (%$hash_ref )) {
        print "$key => $value\n";
    }
    
}
[download]

Actual output:

C_WREN => ARRAY(0x13b2eab0)
CAL_CLK => ARRAY(0x13ad9ea0)
RX_IBIAS_2_25U[4:0] => ARRAY(0x13b2e6a0)
REF_RATE[9:0] => ARRAY(0x13b0e090)
AVD0 => ARRAY(0x13a2f200)
SIDDQ => ARRAY(0x13b0dd00)
[download]

This line:

 $hash{$key_CSV} = \@array_CSV;

Is causing your most immediate problem. You are storing the reference to the array in your hash elements. So when you print out the hash elements, you are getting a reference to an array.