You could easily fix that by adding a counter:

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

 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;
 }
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