use v5.14;

package Foo {
   use Moo;
   has foo1 => (
      is       => "rw",
      init_arg => "foo2",
      reader   => "foo3",
      writer   => "foo4",
   );
}

# Create an object, setting the attribute...
my $obj = Foo->new(foo2 => 40);

# Increment the value...
$obj->foo4( $obj->foo3 + 1 );

# Or cheat and access the hashref directly...
$obj->{foo1}++;

# Print the value of the attribute...
say $obj->foo3;