The first argument of split is either a compiled regexp or a regexp string. | is a special character in regular expressions, so it must be escaped if you mean it literally.

Some solutions:

split(/\|/, ...)
[download]

my $re = qr/\|/;
split($re, ...)
[download]

my $re = "\\|";  # \|
split($re, ...)
[download]

my $str = "|";             # |
my $re = quotemeta($str);  # \|
split($re, ...)
[download]

In your case, the last alternative is the desired one. You want to convert a text into a regexp, and quotemeta will do that for you.

my $sep = $arr[2];
split(quotemeta($sep), ...)
[download]