It would probably help you to work through the examples in perlboot, with attention to the section Invoking an instance method.

When you call your method using $regr->get_mean_x, the instance is provided as an argument. Your method function can use this to access instance variables.

sub get_mean_x
{
    my $self = shift;

    my $raw_dataPtr = $self->{raw_data};
    my $counter = 1;
    my $x_total = 0;
    
    print "raw_datePtr: $raw_dataPtr\n";
    
    foreach my $x (keys %$raw_dataPtr)
    {
        ++$counter;
        $x_total += $x;
    }
    my $mean_x = ($x_total/$counter);
    
    $self->{'mean_x'} = $mean_x;
    
    return ($self->{'mean_x'});
}
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