Maybe you have duplicate keys?

...
my $key= shift @array;
if( $hash{ $key }) {
    warn "Duplicate key '$key'";
};
$hash{ $key }= \@array;
...
[download]

We really need to see the output.   I see a potential problem here in that you are stashing “a reference to @array” ... while that is a single variable.   Hence, all of the references will be to the same block of storage, namely, @array.   In Fortran parlance, all of the references are EQUIVALENCEd.   They will all be seen to contain the last contents of @array, and a change to any one will be reflected in every other, because “you are actually looking at one block of storage, albeit through several mirrors.

I think that you need to be sure that each hash-bucket contains a uniquehashref, and that the values get pushed onto that.   Perl’s “auto-vivification” feature comes in handy, with something like this:

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

my @arry = ( "key", 1, 2, 3 );
my $hash;
my $key = shift @arry;

# NEXT STATEMENT 'AUTOMAGICALLY' CREATES A HASH-ENTRY FOR $key
# AND CAUSES  IT TO CONTAIN AN EMPTY ARRAY IF IT 
# DOES NOT ALREADY EXIST:

push @{ $hash->{$key} }, $_ foreach @arry;

print Data::Dumper->Dump([ \@arry, $hash], 
  ["arry", "hash"] );
[download]

$arry = [
          1,
          2,
          3
        ];
$hash = {
          'key' => [
                     1,
                     2,
                     3
                   ]
        };
[download]

The foreach clause is shorthand for an equivalent loop.   $_ contains the value within each iteration.   Notice how this loop is non-destructive to the content of @arry, iterating through its values without disturbing them.   The magic works now, because we are making copies of each value and pushing those onto a new arrayref (created on-demand) within the hash-bucket for $key.   Each hash-bucket, and each of the values therein, is distinct.

In the statement-of-interest, @{ ... } is part but not all of the magic.   Here, we are telling Perl that the value within the hash-bucket should be interpreted as / initialized to an arrayref.   Perl will automatically create a hash-entry (of course) on demand, because that is what hashes do, but here we’re declaring its type and immediately using it.   We can “auto-vivify” hashrefs, too, so that a line of code something like this ... actually Just Works™:

$hash->{"hickory"}{"dickory"}{"dock"} = "clock";
[download]

$hash = {
          'hickory' => {
                         'dickory' => {
                                        'dock' => 'clock'
                                      }
                       }
        };
[download]

while that is a single variable

Not really, as the variable @array is declared with my inside the loop.
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

Thanks for input. Basically hash will group keys with duplicate values. I replaced with new code by commenting-out warn message with duplicate keys. While in execution, there is error "Global symbol "$hash" requires explicit package name at ./file_name.pl line 69." I don't have hash_ref to hash, to my understanding, without defining hash_ref we can't refer to/and have data written in hash table. Please correct.

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;
        push @{ $hash->{$key_CSV} }, $_ foreach @array_CSV; ++++ Error
+ ++++
        print Data::Dumper->Dump([ \@array_CSV, $hash], ["array_CSV", 
+"hash"] ); ++++ Error +++++
        #if ($hash{$key_CSV})
        #{
        #    warn "Duplicate key '$key_CSV'";
        #};
        #$hash{$key_CSV} = \@array_CSV;
    }
    
    # Explicit scalar context 
    my $size = scalar keys %hash; 
    
    # Open the output file and save hash in $outfile_RX_CSV
    #open (my $outfile2, '>', "$outfile_CSV") or die "Unable to open $
+outfile_CSV: $!\n";
    #print $outfile2 Dumper(\%hash); 
    #close $outfile2;
    #print "Stored $size list of pins in $outfile_CSV file.\n";
    
    # Return a reference to the hash.
    #return %hash;
}
[download]

The syntax for accessing values in a hash via a hash variable is different from the syntax for access values in a hash via a reference to a hash variable. You are using the one where you should be using the other, thus you are getting an error.

To access a value in a hash via a hash variable, you can use the following syntax:

    my $value = $hash{$key};
[download]

To access a value in a hash via a reference to a hash variable, you can use the following syntax:

    my $value = $hash_ref->{$key};
[download]

The -> in this second example dereferences the hash ref.

You have a hash variable (%hash) and a reference to a hash variable ($hash_ref). You don't need the latter, but you can use it if you want to. In fact, you declare $hash_ref and set it to a reference to %hash, but you don't use it anywhere, so you can simply remove it. It makes no difference in the code you posted.

The first line you marked with "++++ Error ++++" includes: $hash->{$key_CSV}. Because $hash is followed by ->, $hash must be a scalar containing a reference, but you don't have such a variable defined in the code you have posted.

If you change that bit to $hash{$key_CSV}, then you will be accessing the value from %hash which you have declared and initialized.

At the second line marked with "++++ Error ++++", you have $hash. This is a scalar variable but in the code you posted it is not declared or initialized. While $hash{$key} accesses a value in the hash %hash, $hash does not refer to %hash at all, it refers to the scalar with a similar name.

The ability to use the same "name" to refer to different variables can be a confusing aspect of Perl. Remember that $name, @name and %name are three different variables: changes to one do not affect the others. It can be more confusing that $name{$key} accesses %name rather than $name, but that's the way it is.

There are many ways you could change the subroutine you posted to eliminate the errors. Here are a couple of simple changes for you to consider:

sub mainCSV
{
    # Open the CSV input file
    open (my $infile_CSV1, '<', "$infile_CSV")
        or die "Unable to open $infile_CSV: $!\n";

    my %hash  = (); 
    
    while (my $line = <$infile_CSV1>) 
    {
        chomp;
        $line =~ s/\s*\z//;
        my @array_CSV = split /,/, $line;
        my $key_CSV = shift @array_CSV;
        push @{ $hash{$key_CSV} }, $_ foreach @array_CSV;
        print Data::Dumper->Dump([ \@array_CSV, \%hash], ["array_CSV",
+ "hash"] );
    }
    
    # Explicit scalar context 
    my $size = scalar keys %hash; 
}
[download]

Or

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;
        push @{ $hash_ref->{$key_CSV} }, $_ foreach @array_CSV;
        print Data::Dumper->Dump([ \@array_CSV, $hash_ref], ["array_CS
+V", "hash_ref"] );
    }
    
    # Explicit scalar context 
    my $size = scalar keys %hash; 
    
}
[download]

And if you really want to do it the second way, you can simplify that to:

sub mainCSV
{
    # Open the CSV input file
    open (my $infile_CSV1, '<', "$infile_CSV")
        or die "Unable to open $infile_CSV: $!\n";

    my $hash_ref = {};
    
    while (my $line = <$infile_CSV1>) 
    {
        chomp;
        $line =~ s/\s*\z//;
        my @array_CSV = split /,/, $line;
        my $key_CSV = shift @array_CSV;
        push @{ $hash_ref->{$key_CSV} }, $_ foreach @array_CSV;
        print Data::Dumper->Dump([ \@array_CSV, $hash_ref], ["array_CS
+V", "hash_ref"] );
    }
    
    # Explicit scalar context 
    my $size = scalar keys %$hash_ref; 
    
}
[download]