I don't have anything to test this on, but it might work to copy from a file handle:

use File::Copy;
{
    open local(*ORIG), '<', '/path/to/encrypted.foo' or die $!;
    copy \*ORIG, '/path/to/plain.foo';
    close ORIG;
}
[download]

That is just a wild guess.

After Compline,
Zaxo