In Perl, how can I replace only the first character of a string?

Question

I don't know the slightest bit of Perl and have to fix a bug in a Perl script.

Given a variable $myvar which contains a string, if the first character is a dot, replace it with "foo/bar".

How can I do this?
(Bonus points if you can guess the bug)

Answer 1

$myvar =~ s+^\.+foo/bar+ ;

Answer 2

You can use substr:

 substr($myvar, 0, 1, "foo/bar") if "." eq substr($myvar, 0, 1);