The 20-year-old signed for Saints on Monday after a successful trial period which came after a belated rejection by Hibs.
Caldwell survived Terry Butcher's cull after Hibs were relegated but the striker then found himself without a club when the Englishman was relieved of his duties ahead of Alan Stubbs' appointment.
Caldwell, who spent the second half of last season on loan at Alloa, told Sky Sports News: "At the end of the season I was offered a one-year extension only for it to be taken away a couple of weeks before I was due to go back, which was really disappointing.
"I was set to go back and it threw my plans up in the air.
"I received a latter from the club and I got a phone call from someone from the club explaining why they had sent the letter.
"They did offer me to go into training to keep up my fitness, which was fair enough."
Caldwell made 27 first-team appearances for Hibs but all but three were off the bench and he is looking for a bigger chance in Paisley.
Caldwell, who netted twice for Hibs, said: "I'm here now and I'm hoping to prove I'm good enough to play in this division and for this team, so I'm looking for a chance to show what I can do.
"I never really got a run at all at Hibs and never played a few games on the bounce.
"This is a fresh start. I feel wanted here and I'm happy to be here and looking to kick on."