Smith (58 not out) and Brad Haddin (35 not out) added a vital 77-run stand for the sixth wicket after Australia lost two quick wickets after lunch to slump to 5-143.
Australia reached 5-220 at tea.
Smith has struck six fours and two sixes off 94 deliveries.
Vice-captain Haddin has been a counter-attacking cornerstone of Australia's Ashes campaign with knocks of 94, 53 and 118 in the first two Tests.
Australia were 3-107 at lunch after choosing to bat first on a bouncy Perth pitch with temperatures in the high 30s.
Chris Rogers (11), Shane Watson (18) and 100-Test veteran Michael Clarke (24) fell in the opening session.
Clarke was caught at short midwicket as spinner Graeme Swann made a crucial breakthrough in the second-last over before lunch.
The Aussie skipper Clarke had been in sparkling touch with centuries in the Brisbane and Adelaide Tests.
Swann, who had taken four wickets in two Tests in a modest start to the series, struck again with the score on 129 when David Warner (60 off 77 balls) played a lame cut shot to backward point.
Warner hit eight fours and one six but was unable to register his fifth Test century and second at the WACA as he fell victim to Swann's extra bounce.
Stuart Broad peppered George Bailey (seven) with three consecutive bouncers, the third of which was top-edged to deep backward square leg in another poor dismissal.
For Bailey, who's averaging 24.25 in the series, it was an embarrassing defeat. The right-hander can expect more bouncers in the second innings.
Swann and Broad have taken two wickets each.
Australia named an unchanged side while England recalled fit-again quick Tim Bresnan for spinner Monty Panesar.
Offspinner Nathan Lyon was retained as Australia opted against an all-pace attack.
"What four quicks can't do, five can't. We need the spinner," Australian coach Darren Lehmann said.
A top of 38 is forecast for Friday with more heatwave conditions over the weekend which will dry out the pitch and possibly cause some cracking.
"It will take some turn as the game goes on," Lehmann said.
Test great and former Aussie gloveman Adam Gilchrist, who scored 17 hundreds, was inducted into the ICC's Hall of Fame and did a motorcade lap of the ground at tea on Friday in front of his home crowd.
Gilchrist is the 19th Australian to be inducted.
AUSTRALIA FIRST INNINGS
C ROGERS run out (Anderson) 11 (9)
D WARNER c Carberry b Swann 60 (77)
S WATSON c Swann b Broad 18 (29)
M CLARKE c Cook b Swann 24 (44)
S SMITH not out 58 (94)
G BAILEY c Pietersen b Broad 7 (12)
B HADDIN not out 35 (53)
Sundries (4lb 2w 1nb) 7
Five wickets for 220
Fall: 13 (Rogers), 52 (Watson), 106 (Clarke), 129 (Warner), 143 (Bailey).
Bowling: J Anderson 9-2-27-0 (1w), S Broad 11-1-54-2 (1w), T Bresnan 14-2-56-0, B Stokes 9-2-34-0 (1nb), G Swann 10-0-45-2.
Batting time: 241 mins. Overs: 53.
Umpires: Billy Bowden (NZL), Marais Erasmus (RSA).