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;