Seems that most of the Perl 6 solutions (never mind the hidious lisp concoctions:) are much clumsier than a fairly obvious Perl 5 solution?

Updated: To correctly handle a null list per TimToady's correction below.

sub p09{
@_?reduce{$a->[-1][0]eq$b?push@{$a->[-1]},$b:push@{$a},[$b];$a;}[[shif
+t]],@_:()
}
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which could almost certainly be golfed further, but for the queasy, expands out to a relatively clean:

use List::Util qw[ reduce ];
sub p09 { 
    @_ ? reduce{ 
          $a->[-1][0] eq $b 
             ? push @{ $a->[ -1 ] }, $b 
             : push @{ $a }, [ $b ]; 
          $a; 
    } [[shift]], @_ 
    :()
}
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