Use a hash to store the id's:
my %id = ( admin => 'admin',
guest => 'guest',
);
my $get_user = 'admin'; # Read this from the config.
my $final_id = $id{$get_user};
my $other_user = 'guest';
my $another_final_id = $id{$other_user};
print $final_id, "\n", $another_final_id, "\n";