While Corion already answered your first question, a link to the appropriate doc would answer your second question. See perldata, specifically the section on scalar value constructors (excerpt follows for convenience):

As in some shells, you can enclose the variable name in braces to disambiguate it from following alphanumerics (and underscores). You must also do this when interpolating a variable into a string to separate the variable name from a following double-colon or an apostrophe, since these would be otherwise treated as a package separator:

    $who = "Larry";
    print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
    print "We use ${who}speak when ${who}'s here.\n";
[download]

Without the braces, Perl would have looked for a $whospeak, a $who::0, and a $who's variable. The last two would be the $0 and the $s variables in the (presumably) non-existent package who.

In fact, an identifier within such curlies is forced to be a string, as is any simple identifier within a hash subscript. Neither need quoting. Our earlier example, $days{'Feb'} can be written as $days{Feb} and the quotes will be assumed automatically. But anything more complicated in the subscript will be interpreted as an expression.

HTH