my @a = qw/x y z
a b c
e f g
h i j/;

my $skip;
for (@a) {
   if ($_ eq "a" && ($skip = !$skip)) {
      splice @a, 0, 0, "#";
   }
}

##</code><code>##

@a = map {$_ eq "a" ? ("#","a") : $_} @a;