Perl needs to know that you want to interpolate $pkg and not $pkg::_ATTR, which would be the variable $_ATTR in the package pkg. Another way to tell Perl to ignore the colons is the following:

@{"$pkg\::_ATTR"} = @_
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which is not necessarily less obfuscated though.

As a small example of the differences, look at the following:

package pkg;
$_ATTR = 'I am $_ATTR from ' . __PACKAGE__;
package main;
$pkg = 'I am $pkg from ' . __PACKAGE__;

print "1. $pkg::_ATTR\n";
print "2. $pkg\::_ATTR\n";
print "3. ${pkg}::_ATTR\n";
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which is not necessarily less obfuscated though.

Yet another way which is, perhaps, a little more clear:

@{$pkg . '::_ATTR'} = @_;
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It's probably expected, though, that anyone reading or changing code using caller and symbolic refs will understand the other forms.

-sauoq
"My two cents aren't worth a dime.";

Thanks, Corion. that helped me a lot. serah

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";
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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