Found in /usr/perl5/5.00503/pod/perlfaq4.pod
Does Perl have a round() function? What about ceil() and floor()? Trig functions?

Remember that int() merely truncates toward 0. For rounding to a
certain number of digits, sprintf() or printf() is usually the
easiest route.

printf("%.3f", 3.1415926535); # prints 3.142

The POSIX module (part of the standard perl distribution)
implements ceil(), floor(), and a number of other mathematical
and trigonometric functions.

use POSIX;
$ceil = ceil(3.5); # 4
$floor = floor(3.5); # 3

In 5.000 to 5.003 Perls, trigonometry was done in the
Math::Complex module. With 5.004, the Math::Trig module (part of
the standard perl distribution) implements the trigonometric
functions. Internally it uses the Math::Complex module and some
functions can break out from the real axis into the complex
plane, for example the inverse sine of 2.

Rounding in financial applications can have serious
implications, and the rounding method used should be specified
precisely. In these cases, it probably pays not to trust
whichever system rounding is being used by Perl, but to instead
implement the rounding function you need yourself.

To see why, notice how you'll still have an issue on half-way-
point alternation:

for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}

0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
0.8 0.8 0.9 0.9 1.0 1.0

Don't blame Perl. It's the same as in C. IEEE says we have to do
this. Perl numbers whose absolute values are integers under
2**31 (on 32 bit machines) will work pretty much like
mathematical integers. Other numbers are not guaranteed.