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

Update: Here's a version that is a little more efficient, removing one boolean comparison from the loop.

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;
}
[download]