Email really only deals with the ASCII charset. As a result anything outside of this charset really needs to be base 64 encoded. You are already torturing the mailserver by sending non ascii chars in the body (ie e acute). It may be your CSV contains non ASCII chars and this is causing the mail server to barf.
However, the problem is probably this:
# needs to be all on one line or like this:
The whitespace before boundary indicates that is the line continuation (it is), but in your code you send it as a header line (it isn't). It does not really explain your symptoms as described, but is wrong and should fail.
As you seem hell bent on rolling your own solution I suggest you base64 encode your CSV and send it as binary. You can just cut and paste the < 30 lines of code from MIME::Base64 encode routine to do this. BTW I would not use just "frontier" as your multipart boundary. "==frontier==", which will become "--==frontier==" would seem a far more improbable boundary.
You can make your code a lot more readable if you use a HEREDOC.
my $body = ....
my $attach = .... # base 64 encoded data
my $a_name = .....
my $email =<<EMAIL;
Content-Type: multipart/mixed; boundary="==frontier=="
This is a multi-part message in MIME format.
Content-Type: application/octet-stream; name="$a_name"
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