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.