...
my $pw;
do {
   $pw = make_new_pw();
} while ( grep { $pw eq $_ } @stored );
...

# update: moved the declaration of @chars into the subroutine,
# where it belongs:

sub make_new_pw
{
    my @chars = ( "A" .. "Z", "a" .. "z", 0 .. 9, qw( ! @ $ % ^ & *) );
    my $newpw = "";
    for ( 1 .. 17 ) {
        $newpw .= $chars[ rand @chars ];
    }
    return $newpw;
}