This seems to be what you want, albeit not very clever - it will fail if the message contains the word "at". :)

use warnings;
use strict;

while (<DATA>) {
    my ($msg, $line) = /((?:(?!\bat\b).)*)(.*)/;
    print "$msg\n   $line\n";
}

__DATA__
This is my text at noname2.pl line 4.
[download]

Prints:

This is my text 
   at noname2.pl line 4.
[download]