Also, note that $self is kind of reserved (although not enforced) for a blessed object. The way you're assigning $self is and will be very confusing to people familiar with OO programming in Perl. If you are not using OO, don't use $self as the variable name. Here's an example using $self in proper context:

use warnings;
use strict;

package Blah; {

    sub new {
        my ($class, $params) = @_;
        my $self = bless $params, $class;
        return $self;
    }
    sub display {
        my ($self) = @_;

        # should be using getters, but I digress

        for (keys %{ $self->{params} }){
            print "$_: $self->{params}{$_}\n";
        }
    }
}

package main; {

    my $hashref = {
        num1 => 1,
        num2 => 2
    };

    my $obj = Blah->new({params => $hashref});
    $obj->display;
}
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