A bug of sprintf or not?

[pysome]

I found a odd question when i wanna round a float number
with Perl buildin function "sprintf" my code is

use warnings;
use strict;

my $price1   = sprintf("%.1f",100.550);
my $price2   = sprintf("%.0f",100.50);
print "$price1\n";  # price1  has rounded
print "$price2\n";  # but price2 hasn't , Why?

Of course, i can use the manual round function like some referrence:

sub round {
  my $f = @_;
  return int($f + .5);
}

But the question is Why $price1 has been rounded and price2 has not been rounded?

[ikegami]

Anonymous Monk is mistaken. This isn't a precision issue. It's banker's rounding.

printf("%.0f\n", 100.5);  # 100
printf("%.0f\n", 101.5);  # 102
printf("%.0f\n", 102.5);  # 102
printf("%.0f\n", 103.5);  # 104
printf("%.0f\n", 104.5);  # 104
printf("%.0f\n", 105.5);  # 106

Note that all those numbers (but not 100.55) can be represented precisely in floats (0.5 = 2^-1).

[Anonymous Monk]

Simple math gone wrong (What Every Scientist Should Know About Floating-Point Arithmetic)