my $x = 8.78; 
my $y = $x * 100;
my $z = $x * 1000;

printf "%08d\n", $y;     # prints 00000877 - wrong
printf "%08d\n", "$y";   # prints 00000878 - right 
printf "%3.5f\n", $y;    # prints 878.00000 - right
printf "%08d\n", $z;     # prints 00008780 - right