First of all, you are not looking for the nearest value. What you need is a customised ceil (see perldoc POSIX).

In such cases, where you want to look for the "nearest but not smaller then" and your step is not 1, but X (X = 50 in your case), you can do the following:

use POSIX;

sub ceilX ($$);

print "$_ -> ", ceilX($_, 50), "\n" for (145..154);

sub ceilX ($$) {
    my $number = shift;
    my $x      = shift;

    return POSIX::ceil($number / $x) * $x;
}
[download]

with the result

145 -> 150
146 -> 150
147 -> 150
148 -> 150
149 -> 150
150 -> 150
151 -> 200
152 -> 200
153 -> 200
154 -> 200
[download]

Edit: Sorry, I was interrupted while writing this answer, when I started writing, there was no comparable posted.