Ajusted to only do it once:

my $first = 1;
@array = map {
   if ($first && $_ eq "c") {
      $first = 0;
      ($_, "x")
   } else {
      $_
   }
} @array;
[download]

hum, ugly. How about:

my $first = 1;
@array = map {
   my @a = $_;
   push(@a, "x") if ($first && $_ eq "c");
   $first ||= @a-1;
   @a
} @array
[download]

Nope, still ugly. Well, you could always do:

foreach (0..$#array) {
   if ($array[$_] eq "c") {
      splice(@array, $_+1, 0, "x");
      last;      
   }
}
[download]