I was thinking along lines similar to Marshall:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"use constant {
   START => 6,
   STOP  => 7,
   };
 ;;
 my $aref = [ 6, 7, 1, 6, 8, 9, 7, 2, 6, 98, 99, 7, 3, 7, 4, 6, 5, ];
 dd $aref;
 ;;
 my @final;
 my $maybe_truncate;
 ;;
 for my $element (@$aref) {
   if (defined $maybe_truncate) {
     if ($element == STOP) {
       $#final = $maybe_truncate;
       undef $maybe_truncate;
       next;
       }
     }
   else {
     if ($element == START) {
       $maybe_truncate = $#final;
       }
     }
   push @final, $element;
   }
 ;;
 dd $aref;
 dd \@final;
"
[6, 7, 1, 6, 8, 9, 7, 2, 6, 98, 99, 7, 3, 7, 4, 6, 5]

[6, 7, 1, 6, 8, 9, 7, 2, 6, 98, 99, 7, 3, 7, 4, 6, 5]

[1, 2, 3, 7, 4, 6, 5]
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I suspect this logic can be cleaned up significantly. I'd like get something approaching tybalt89's solution.