TMTOWTDI. Here’s a general approach using sprintf:

#! perl
use strict;
use warnings;
use Test::More;

while (<DATA>)
{
    my ($decimals, $source, $target) = / (\d+) \D+ ([\d.]+) \D+ ([\d.]
++) /x;

    my  $result = round($source, $decimals);

    is($result, $target, "round $source to $target");
}

done_testing();

sub round
{
    my ($num, $decimals) = @_;

    return sprintf("%.*f0", $decimals, $num);
}

__DATA__
1 1.24 to 1.20
1 13.25 to 13.30
1 2.90 to 2.90
1 2.99 to 3.00
1 2.92 to 2.90
1 14 to 14.00
5 1.2345678 to 1.234570
[download]

Output:

16:05 >perl 1544_SoPW.pl
ok 1 - round 1.24 to 1.20
ok 2 - round 13.25 to 13.30
ok 3 - round 2.90 to 2.90
ok 4 - round 2.99 to 3.00
ok 5 - round 2.92 to 2.90
ok 6 - round 14 to 14.00
ok 7 - round 1.2345678 to 1.234570
1..7

16:05 >
[download]

Hope that helps,