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PenFal opened the scoring following a sustained series of pick-and-goes close to the St Austell try line, with Sam Winfrey eventually powering over.
Brodie Creeden added the conversion for an early 7-0 advantage.
A second try soon followed through Toby Thomas, with Brodie Creeden again successful from the tee to double the lead to 14-0.
PenFal's third try was arguably the pick of the afternoon.
Following a missed lineout, an aggressive defensive effort drove St Austell backwards and forced a clearance kick into the PenFal half.
From there, the home side launched an excellent counterattack, combining powerful carries with quick recycling and strong support at the breakdown.
The ball was eventually released to the backs, where slick handling created the space for Noah Carpenter to finish in the corner.
Brodie Creeden's conversion stretched the lead to 21-0.
Conan Hitchens then crossed for the bonus-point try, with another Brodie Creeden conversion making it 28-0.
St Austell responded with an unconverted try of their own, but PenFal quickly regained control.
Before the interval, Conan Hitchens added his second try of the afternoon, while Aiden Jones and Charlie Mount also crossed as a dominant first-half display gave PenFal a commanding 47-5 lead at half-time.
Credit to St Austell, who responded strongly after the break and edged the second half 7-5.
Ollie Brown scored PenFal's only try of the half, but the damage had already been done by the home side's impressive opening 40 minutes, with the match finishing 52-12.
The Forward of the Match award went to Sam Winfrey, who produced another excellent performance with powerful ball carrying and strong tackling throughout.
Back of the Match award went to Conan Hitchens, whose two tries were complemented by some excellent support play and work alongside his wingers.
The victory takes PenFal to a 4-0 league record, continuing an excellent unbeaten start to the season.