Better how?

Shorter? Put the code in a sub.

More readable? Put the code in a sub.

Faster? Sort the indexes and use a merge sort-like approach written in C to filter out the unwanted elements.

Alternative approach if the array only has defined elements:

my @invslice = @array;
@invslice[ @slice_idx ] = ();
@invslice = grep defined, @invslice;
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Update: Replaced undef @invslice[ @slice_idx ]; (which doesn't work) with @invslice[ @slice_idx ] = ();.