You can mill your own transition strips using hardwood lumber and a tablesaw.
Finish carpet in doorway.
Use your utility knife to cut the carpet to the size of the room adding an extra 4 to 6 inches on each side.
Place a straightedge on top of the carpet and cut along the line using a utility knife to trim off the excess.
Just push the edge of the carpet down into the gap all the way across the door and trim any strings off.
But when the last few floorboards of your laminate floor need to be laid under the door jambs or through the door lining or door frame it seems impossible to get them in without.
Lay your carpet down and make sure it is centered and lays directly against the doorway.
If you nail one down always nail it to the subfloor and not to the pergo flooring.
If have to butt pergo against an existing metal track for a sliding door leave a 1 4 inch gap between the doorway and the floor and fill the gap with acrylic latex or silicone caulk.
After the carpet is stretched onto the tack strip and cut to the correct size it needs to be tucked down into the gap.
I use a hook bill to do this.