rename "$file_in", "$random_num"."$file_in" or warn "Couldn't rename file: $!\n";

##</code><code>##

if (-e $file_in) {
    rename($file_in, "$file_in.$random_num");
}

##</code><code>##

my $f = $file_in;
while (-e $f) {
    $f = "$file_in." . int(rand(100000));
}
rename($file_in, $f);