Your reasoning is sound. I have only a small remark. If you want to print the two numbers to show their differences, increase the number of decimals.
#!/usr/bin/perl -w
use strict;
my ( $number, $premium, $expected );
$number = 1.80;
$premium = $number * ( 1 + 10/100 ); # 1.8 + 10% of 1.8
$expected = 1.98; # As we know 1.8 + 10% of 1.8 is 1.98
printf "Number 1 : %20.19f\n", $premium ;
printf "Number 2 : %20.19f\n", $expected ;
print "Not" if ( $expected != $premium );
print "Equal !! ";
__END__
Number 1 : 1.9800000000000002043
Number 2 : 1.9799999999999999822
NotEqual !!
