The Italian laws are codified into several different "codes" based on their topic, including those relevant here: criminal procedural law (Codice di procedura penale, CPP) and criminal law - that is, a list of crimes and their penalties (Codice penale, CP).
Calunnia is defined as a crime, so it is included, along with other crimes, for example, murder and fraud by the authorities in an investigation or trial, in the CP. It does not belong in the CPP, which has laws specifying procedures, for example, the warnings by the police or prosecutor that must be given to a suspect under interrogation and the procedures to be followed in a first-instance trial, in an appeal trial, and in an appeal to the CSC.
I have previously posted the text of the crime of calunnia in Italian and in English translation. There are several online sources that have the CP and the CPP (as well as the other codes) in Italian. Here's a list of a few of those sources, listing the CP page (in most cases there's a link to the CPP from that page); the last source is from the "Normattiva" ("Standards of Law") of the CSC:
https://www.altalex.com/documents/codici-altalex/2014/10/30/codice-penale
https://www.brocardi.it/codice-penale/
https://www.studiocataldi.it/codicepenale/codicepenale.pdf
https://www.normattiva.it/uri-res/N2Ls?urn:nir:stato:regio.decreto:1930-10-19;1398
Here's the Italian text of the law against calunnia, from the 1st source; the footnotes are explanations of changes to the law:
Here's a translation (Google with my help using Collins Reverso & Gialuz et al.):
Art. 368.
False Accusation.
Anyone who, with a report, complaint, request or application, even if anonymous or under a false name, directs to the judicial authority {police, prosecutor, or judge} or to another authority that is obliged to report to it or to the International Criminal Court, accuses anyone he knows to be innocent of a crime, or simulates evidence of a crime against a person he knows to be innocent, is punished with imprisonment of from two to six years (1).
The penalty is increased if someone is accused of an offense for which the law establishes the penalty of imprisonment exceeding a maximum of ten years, or another more serious penalty.
The imprisonment is from four to twelve years, if the act of the alleged crime results in a prison sentence of more than five years; is from six to twenty years, if the act results in a life sentence; and the penalty of life imprisonment is applied, if the act results in a death penalty (2).
(1) Paragraph thus amended by art. 10, paragraph 3, Law 20 December 2012, n. 237. The text previously in force was the following: "Anyone who, with a report, complaint, request or application, even if anonymous or under a false name, addressed to the judicial authority or to another authority which is obliged to report to it, accuses anyone he knows to be innocent of a crime, or simulates evidence of a crime against the person he knows to be innocent, is punished with imprisonment of from two to six years. ".
(2) The death penalty for crimes provided for by the penal code has been abolished by art. 1 of the Legislative Decree 10 August 1944, n. 224.
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ETA: One of the crimes the Italian prosecutors agreed was implied as having been committed by the police and Mignini by Amanda Knox in her testimony before the Massei court, and in her appeals, was calunnia. In other words, if Amanda Knox's testimony about the interrogation is true, the Italian authorities believe that the case can be made that the police and Mignini framed her by means of an intentional (malicious) false accusation. Similarly, the police and Mignini would have committed the crimes of falsification of official documents and (IIRC) the use of threats and violence to obtain false testimony. This information may be found in the introductory part of the Boninsegna court motivation report.