Stepping through an array and making changes "as-you-go" has unforeseen problems. We get quite a few "Why didn't this work?" questions related directly to attempting to do this type of thing.

The issue is actually documented in "perlsyn: Foreach Loops" and specifically mentions splice:

"If any part of LIST is an array, foreach will get very confused if you add or remove elements within the loop body, for example with splice. So don't do that."

[In case you didn't know, for and foreach are synonymous.]

So, you'd need to do something like the following.

sub splice_unique2 {
    my @sorted = sort qw{q w e r t y q w e r t y};
    my $last = '';
    my @unique = @sorted;
    my $removed = 0;

    for (0 .. $#sorted) {
        if ($sorted[$_] eq $last) {
            splice @unique, $_ - $removed++, 1;
        }
        else {
            $last = $sorted[$_];
        }
    }
}
[download]

I plugged that in to my earlier benchmarking script. Here's a representative result:

                   Rate splice_unique2 splice_unique  shift_unique    
+map_unique
splice_unique2  83836/s             --          -10%          -15%    
+      -28%
splice_unique   92839/s            11%            --           -6%    
+      -20%
shift_unique    99211/s            18%            7%            --    
+      -14%
map_unique     115924/s            38%           25%           17%    
+        --
[download]

My coding of splice_unique2() was really just intended to show a way to get around the documented issue. If you think the task has any merit, feel free to try to rewrite it to run faster.

Thank you Ken, will read up on it :)