There are a few things you will want to change with this code.

• Using @_ directly is ugly and hard to decipher, in my opinion. Consider assigning the parameters to a list of variables like so: my ($class, $rawData) = @_ instead of using $_[1].
• There is no implicit $self in methods like there are in other languages. $self is the first argument passed to the method. You will often see a my $self = shift as the first line in a method.
• You're missing a few my declarations and there are some typos, so your example doesn't compile.

Overall the code you need is there. With a few tweaks you should end up with something working like this:

use strict;

package regression;

sub new
{
    my ($class, $raw) = @_;
    
    bless
    {
        'raw_data'    =>     $raw,
        'mean_x'    =>    0,
        'mean_y'    =>    0,
        'slope'        =>    0,
        'y_intercept'    =>    0
    }, $class;
}

sub get_mean_x
{
    my $self = shift;
    my $raw_dataPtr = $self->{'raw_data'};
    my $counter = 1;
    my $x_total = 0;
    
    for my $x (keys %$raw_dataPtr)
    {
        ++$counter;
        $x_total += $x;
    }
    
    $self->{'mean_x'} = $x_total / $counter;
}

package main;

my ($mean_x,$mean_y,$slope,$y_intercept);
my %raw_data = (
    1    =>    100,
    2    =>    160,
    2.5    =>    182,
    3    =>    225
);

my $regr = regression->new(\%raw_data);
$mean_x = $regr->get_mean_x;
print "Mean_x: $mean_x\n";
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