multiline mode in Perl and Ruby different: Ruby is wrong?

Question

Let us look at Perl code and result:

$s = "a\\nb\\nc\\n";
$s =~ s/^b/X/;
print $s;

a
b
c

$s = "a\\nb\\nc\\n";
$s =~ s/^b/X/m;
print $s;

a
X