#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;
#http://www.perlmonks.org/?node_id=1193467

my $aref = [1, 7, 6, 2, 2, 7, 1, 6, 99, 99,7,88, 7, 6,1,2,3];

my @result;
my @stack = ();

foreach my $candidate (@$aref)
{
   if ($candidate == 6)   # starting flag
   { 
      push @stack,6;
   }
   elsif ($candidate == 7) #potential ending flag
   {
      if (@stack)
      {
          @stack=(); #throw away between 6, x, y, 7
      }
      else
      {
          push @result, 7;  # a singleton 7 was seen
      }
   }
   elsif (@stack) # inside of sequence starting with 6 
   {
      push @stack, $candidate;
   }
   else
   {
      push @result, $candidate;
   }
}

push @result, @stack if @stack; # unfinished sequence starting with 6?
 

print Dumper \@result;

__END__
prints:
$VAR1 = [
          1,
          7,
          1,
          88,
          7,
          6,
          1,
          2,
          3
        ];