You could do 

for (0..(5-1)/2) {  # 1..5, step 2
   my $i = $_*2 + 1;
   ...
}

[download]

but you're probably better off falling back to a C-style loop in that case.

As an aside, the above trick is particularly useful when dealing with floats. 

>perl -le "for (my $i=0; $i<10; $i+=0.1) { print $i }"
...
8.79999999999998
8.89999999999998
8.99999999999998
9.09999999999998
9.19999999999998
9.29999999999998
9.39999999999998
9.49999999999998
9.59999999999998
9.69999999999998
9.79999999999998
9.89999999999998
9.99999999999998

>perl -le "for (0..99) { my $i = $_ / 10; print $i }"
...
8.8
8.9
9
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9

[download]

In reply to Re^3: optimizing the miller-rabin algorithm by ikegami
in thread optimizing the miller-rabin algorithm by punklrokk

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