Search for unique, you'll find plenty of references, as this question comes up fairly often.

update: Well most of those nodes discuss how to create a completely unique list, which is not exactly what you want, so here:

update2: Fixed some yucky code... =)

my @in = (1,2,3,4,4,4,5,5,6,7,8,9,9,1,2,3,3);

## one way

my @out1 = $in[0];
do { push @out1, $_ if $out1[$#out1] ne $_ } for @in;

## or another way

my $t = '';
my @out2 = grep { $t eq $_ ? 0 : ($t = $_, 1) } @in;
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Of course, there are many ways to slice this particular cat...

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print