To make print treat an expression in parenthesis as an argument (and not as the argument list) just put a '+' in front of the parenthesis' opening:

for my $i (1..20) {
  print "$i: ";
  print+(1-1/2000)**$i;
  print " - ";
  print+(1-1/2000)**int($i);
  print " - ";
  print "" . (1-1/2000)**$i;
  print "\n";
}
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