man 1 passwd

--stdin
          This option is used to indicate that passwd should read the new
          password from standard input, which can be a pipe.

adduser "$1"
echo "$2" | passwd "$1" --stdin

#!/usr/bin/perl -w
open my $pipe, '|chpasswd' or die "can't open pipe: $!";
print {$pipe} "$username:$password";
close $pipe

echo $mypassword | passwd --stdin # Eternal Sin.
echo "$mypassword" | passwd --stdin # Eternal Sin, but at least you remembered to quote your PE.
passwd --stdin <<< "$mypassword" # A little less insecure, still pretty insecure, though.
passwd --stdin < "passwordfile" # With a password file that was created with a secure `umask(1)`, a little bit secure.

user="$1"
password="$2"
adduser $user
echo $password | passwd --stdin $user