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 !!
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