If a variable were overwritten by a crazy, deranged or plain curious person, could that have more serious effect than stop the code from working?

Potentially, if crafted in malice, depending on what your code does with global variables. (Bear in mind also that the special variables are vulnerable under your implementation.) You could think through each and every global variable and each special variable to determine whether anything your script does could have bad effects if one of these variables holds a malicious value, or you could store the input in a hash and save yourself that effort. Running under taint mode would also help to curb this threat or at least make it much harder for anyone to exploit.

$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/
[download]