Anonymous Monk,
I am sure there is a more elegant way than this and that some other uber cool monk will provide it, but here goes:

#!/usr/bin/perl -w
use strict;

my @array = qw(0 1 2 3 4 5 6 7 8);

print join '_' , grep length, map {$array[$_] if $_ % 2 == 1} (0 .. $#
+array);
[download]

Cheers - L~R

Update: If you know the number of elements in the array, don't use this method - use BrowserUk's solution