Probably the easiest way is to use printf/sprintf:

$unrounded = 0.66666;

printf "$unrounded rounded to 3 decimal places is %.3f\n", $unrounded;

$rounded = sprintf "%.2f", $a;  # rounded to 2 decimal places (0.67)
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As in How do I round a number? — Math::Round has a great method for this:

use Math::Round;

print nearest(.01, 1.555);
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This other answer, by wrvhage, has the great property of working correctly for values like 3.005 rounded to two places (where sprintf is not). However, it needs a couple of tweaks to work for negative numbers.

sub stround
{
    my( $n, $places ) = @_;
    my $sign = ($n < 0) ? '-' : '';
    my $abs = abs $n;
    $sign . substr( $abs + ( '0.' . '0' x $places . '5' ), 0, $places 
++ length(int($abs)) + 1 );
}
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sub stround
{
    my( $number, $decimals ) = @_;
    substr( $number + ( '0.' . '0' x $decimals . '5' ), 0, $decimals +
+ length(int($number)) + 1 );
}
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sub stround
{
  my ($n, $places) = @_;
  my $abs = abs $n;
  my $val = substr($abs + ('0.' . '0' x $places . '5'),
                   0,
                   length(int($abs)) +
                     (($places > 0) ? $places + 1 : 0)
                  );
  ($n < 0) ? "-" . $val : $val;
}
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sub round {

    my ($nr,$decimals) = @_;
    return (-1)*(int(abs($nr)*(10**$decimals) +.5 ) / (10**$decimals))
+ if $nr<0;
    return int( $nr*(10**$decimals) +.5 ) / (10**$decimals);

}
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Originally posted as a Categorized Answer.