Rounding without mathematical operations will be far slower. Treating numbers like strings in order to round them is probably a dubious task. Use a mathematical approach:

# $rounded = round($value, $places);
sub round {
  my ($N,$P) = @_;
  my $pm = $N > 0 ? 1 : -1;
  return int($N * 10**$P + .5 * $pm) / 10**$P;
}
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