As already stated in the OP the need to appropiately name a whole new family of reduce-like iterators is an extra problem.

Your always free to not give it a name, leaving it as

my @b;
for (@a) {
   push @b, $_ if !@b || $_ ne $b[-1];
}
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my @b = list_reduce { 0, $a || $b ne $_->[-1] ? $b : () } 1, @a;
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But they're not very readable.

But I'd prefer to have a solution where map is still named map and grep ist still grep

You complain that there's "a lack of iterators allowing to compare successive elements", yet you wouldn't want to use them if they existed?