I suggest structuring your if/elsif/else structure differently, to reflect that there are nested conditions, and this will make the code a little more readable. Using numerical operators will help. Beware of use of > when >= may be needed.

if ($downpercent == 20)
{
  $totalpmt = $retioincome33 > $retiodebt38 ? $ratiodebt38 : $ratioinc
+ome33;
}
else
{
  $totalpmt = $ratioincom28 > $ratiodebt36 ? $ratiodebt36 : $ratioinco
+me28;
}
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