"it will prevent these kind of error if future"

If you want to prevent errors be aware of perlop "Operator precedence and associativity".

use strict;
use warnings;

my $avail = 1;
my $used  = 1;

my $testa = ( $avail / $avail + $used ) * 100;
my $testb = ( $avail / ( $avail + $used ) ) * 100;

print "testa: $testa\ntestb: $testb\n";
[download]