@array = map { $_ eq "c" && !$found++ ? ("c","f") : $_ } @array

##</code><code>##

 sub insert_after_first {
   my $arr_ref = shift;
   my $to_insert = shift;
   my $insert_after_me = shift;

   my $found = 0;

   map { $_ eq $insert_after_me && !$found++ ? ( $insert_after_me, $to_insert ) : $_ } @$arr_ref;
 }