I added some test cases.

• I assume that some sequence that starts with 6 but never has a "terminating" "7" means that those numbers appear in the result.
• sequence starts on the "first 6" and ends on "next 7" (if it ends at all, see previous)
• A "7" that occurs without a previous matching "6" is part of the results

I wasn't sure if using splice was an absolute requirement, so I didn't use it. Also, as a general rule, I try to avoid using indicies. Code using indicies tends to be more error prone due to the dreaded "off by one" error which even very experienced programmers are prone to make.

In the code below, once a 6 is seen, I keep the numbers on a stack so that I can use them later if no matching 7 is found. The scalar value of @stack also does double duty as the "seen a 6 - inside of a potential sequence flag". I think the logic here is clear albeit a bit wordy. However well spaced out, simple code tends to run very quickly and I don't consider a bunch of white space a "problem"

#!/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
        ];
[download]

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