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    my $value = $hash{$key};

    my $value = $hash_ref->{$key};

sub mainCSV
{
...
    # Explicit scalar context 
    my $size = scalar keys %hash; 
}

sub mainCSV
{
...
    my $size = scalar keys %hash; 
    
}

sub mainCSV
{
...
    my $size = scalar keys %$hash_ref; 
    
}