sub insert_after_first {
    my ($ndx, $arr, $elt, $ins) = (0, @_);
    $ndx > @$arr and return while $arr->[$ndx++] ne $elt;
    splice @$arr, $ndx, 0, $ins;
}

##</code><code>##

sub insert_after_first {
    my ($ndx, $arr, $elt, $ins) = (0, @_);
    $arr->[$ndx++] eq $elt and last for 0 .. @$arr;
    splice @$arr, $ndx, 0, $ins if $ndx <= @$arr;
}