It's much simpler than the "semipredicate problem" because all you have to do is set the previous value to "not the first value".

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=880228
use warnings;

sub squeeze
  {
  no warnings; 
  my $prev = not $_[0];
  return grep +($_ ne $prev, $prev = $_)[0], @_;
  }
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