This is possible to solve using map. map removes list the element in question if an empty list is returned: @after = map { (length) ? $_ : () } @before; FYI, you can also return lists to insert entries. From perlfunc under map:

Evaluates BLOCK or EXPR in list context, so each element of LIST may p
+roduce zero, one, or more elements in the returned value.
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However, grep is more suitable for this purpose. It's also faster:

use Benchmark;

@b = (1, 2, 3, undef, 4, 5, undef, 5, 6, 7, undef, 8,
     undef, 9, 0, 0, 0, 0, 1, 3, 4, undef, undef, undef, 9);
timethese(50_000, {
    'map' => sub
    {
        my (@a);
        @a = map { (length) ? $_ : () } @b;
    },
    'grep'=> sub
    {
        my (@a);
        @a = grep { length } @b;
    }
});


Benchmark: timing 50000 iterations of grep, map...
      grep:  6 wallclock secs ( 6.48 usr +  0.00 sys =  6.48 CPU) @ 77
+16.05/s (n
=50000)
       map: 10 wallclock secs ( 9.28 usr +  0.00 sys =  9.28 CPU) @ 53
+87.93/s (n
=50000)
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