for ($i=0;$i<10;$i+=.5){
    printf "%.1f: %.0f\n", $i, $i;
}
__END__
Output:
0.0: 0
0.5: 0
1.0: 1
1.5: 2
2.0: 2
2.5: 2
3.0: 3
3.5: 4
4.0: 4
4.5: 4
5.0: 5
5.5: 6
6.0: 6
6.5: 6
7.0: 7
7.5: 8
8.0: 8
8.5: 8
9.0: 9
9.5: 10