If Italy declared war on the allies in 1939, I would expect some major repercussions/butterflies.
It would derail France’s reinforcement plans from North Africa. Those troops might have been necessary for the African defence and/or their transportation from NA to France could have been interdicted by the Italian navy. At the very least, that possibility would be high on everybody’s mind. The non-appearance of the hard-fighting African troops in France would instil caution in the French high command, which already feared the larger numbers of the German army (as they perceived it).
I also don’t believe France and England would invade Libya and destroy the Italian colonial empire. Italy had a vast army in its colonies and the utter uselessness of the Italian army was not yet known. France had spent a fortune on building the Mareth line in Tunisia, so a defensive stance would be the most likely action. Meanwhile, the British also lacked the forces to successfully invade Libya. Historically, it only skirmished with light recon forces until the Italians invaded Egypt and Wavell had assembled enough forces to counter-attack in what had become England’s main theatre of war after being expelled from the mainland. With war in the west and France menaced from the North-East and South-East, there would be little manpower to waste on ancillary operations.
The main difference with OTL may well have been the Greek adventure and the Russian Expeditionary Corps/8th Army. Historically, Mussolini was an egomaniac and opportunist. He invaded Greece because he felt slighted and wanted an easy victory. He also sent his best troops to Russia, in support of Fascist cooperation.
Had he not done that and focused on Africa (which would be likely if he was already at war IMO), he should have been able to defeat the meagre British forces in the region in 40-41 before they could build up a larger army.
Instead, Mussolini flittered his troops away in multiple campaigns, as a child with a low attention span, never bothering to prepare for a campaign or see it through to the end.