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# Instead of passing a file name, a malicious user sends
# another command
...

# system() happily executes "ls -l" followed by "rm -rf /"
system("ls -l $user_input");

$user_input = "; rm -rf /";

...
# directory" error
system("ls", "-l", $user_input);

# Bad
@output = `ls -l $user_input`;
...
        die "could not fork" if !defined($kidpid);
        exec ("ls", "-l", $user_input) or die "exec failed: $!";
}