The following does what you want, but it may be over-fussy:

use strict;
use warnings;

my @files = split '@@', <DATA>;

s/^.*?c:\\temp// for @files;
@files = grep {/^\\/} @files;
print join "\n", @files;

__DATA__
c:\temp\source\test\test.c@@main\com\etc c:\temp\source\test\test1.c@@
+main\com\test c:\temp\source\test\test3\test2.c@@main\com\etc\test
[download]

Prints:

\source\test\test.c
\source\test\test1.c
\source\test\test3\test2.c
[download]