While I can't tell you why your server would reboot when your script die'd (although I don't think you want to be die'ing in a CGI)... your

open FILE, "file" || die "Cannot open $file";
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will NEVER die, because || has higher precedence. Thus, it's actually doing

 open FILE, ("file" || die "Cannot open $file");
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The parentheses you had in before prevented the || from being evaluated first.