sub hoge_r {
    my $s = shift;
    substr($$s, 1, 1) = "1";
}

sub hoge {
    my $s = shift;
    hoge_r(\$s);
}

my $a = "abc";
my $b = $a;
hoge($a);
print $a."\n";

hoge_r(\$a);
print $a."\n";
print $b."\n";