The easy way to do this is with Moose:

package Obj;

use Moose;

has 'a', is => 'ro', default => 10;

package main;

my $obj = Obj->new( a => 10 );
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... though if you must write your own initializer, the easiest and clearest and probably most efficient way is:

sub initialize
{
    my $self = shift;

    while (my ($key, $value) = splice @_, 0, 2))
    {
        $self->{$key} = $value;
    }
}
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