@array = do {
  my @array2;
  foreach ( @array ) {
    push @array2, $_;
    push @array2, "new_element" if $_ eq 'remove';
  }
  @array2;
};


##</code><code>##

@array = map { $_, ( $_ eq 'remove' ? "new element" : () ) } @array;

##</code><code>##

@array = map {    $array[$_], (  ($_+1) % 3 == 0  ?  "new element"  :  ()  )    } 0 .. $#array;