Normally, you shouldn't need the number at all:

my @dirs = ('/path/to/dir1', '/some/other/path/to/dir2', '/dir3');
for my $dir (@dirs) {
    ...
}
[download]

To populate the array with the names, you can use a map:

my @dirs = map { sprintf 'abc_%03d', $_ } 1 .. 100;
[download]

If you really need the array index, loop over the indices rather than the elements themselves:

for my $i (0 .. $#dirs) {
    my $dir = $dirs[$i];
    ...
}
[download]