How Do I iterate through the array, yet compare the next item against the former?

You use an index, or you keep a stack

The index approach, its missing boundary checking ( ; beware of off-by-one errors

my @stuff = 0 .. 9;
for my $ix ( 0 .. $#stuff ){
  my $item = $stuff[$ix];
  my $prev = $stuff[$ix-1];
  my $next = $stuff[$ix+1];
}
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The stack approach

my @stuff = 0 .. 9;
my $lastitem = "";
for my $item ( @stuff ){
  my $next = $item;
  if( $item eq $lastitem ){
    print "they measure up\n";
  }
  $lastitem = $item;
}
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Oh, but you $lastitem is not an array, right

my @stuff = 0 .. 9;
my @lastitem ;
for my $item ( @stuff ){
  my $next = $item;
  if( @lastitem and  $item eq $lastitem[-1] ){
    print "they measure up\n";
  }
  push @lastitem, $item;
  shift @lastitem while 3 == @lastitem; ### keep at most 3 last items
}
[download]

And I didn't preview enough :) or test the logic   shift @lastitem while  @lastitem > 3; ### keep at most 3 last items