You probably don't need binmode unless your script is running in Windows. I doubt it is because you referenced /tmp. If that is the case, comment out the binmode lines below. When working with binary files, use read() to have better control over memory usage.

open (IN, "<$path/$filename.tmp") || die $!;
binmode IN;
open (OUT, ">$path/$filename.jpg") || die $!;
binmode OUT;
print OUT $buf while read IN, $buf, 8192;
close IN;
close OUT;
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P.S.
If you are transfering that file via FTP from the server to the windows box to test opening it . . you did remeber to set FTP to binary more by issuing "bin" before the get right?