See also What Every Computer Scientist Should Know About Floating Point. And consider the following variation of your program:

use strict;

my $ammount;
my $value;
my $response = qq{Value should be 189.81 (rounded to two decimals)} .
               qq{ and sprintf gives us: %.24f \n};

$ammount = 159.5;
$value   = $ammount * 1.19;
print sprintf($response, $value);

$ammount = 159.5;
$value   = ($ammount*119)/100;
print sprintf($response, $value);
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Which produces

Value should be 189.81 (rounded to two decimals) and sprintf gives us:
+ 189.804999999999978399500833
Value should be 189.81 (rounded to two decimals) and sprintf gives us:
+ 189.805000000000006821210263
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You can clearly see from this output why sprintf rounds the two values differently.