Calling Foo::new() calls the function new in the package Foo as a direct function call. It doesn't magically put the class name in the first element of @_ in Foo::new(). The call is completely different. This example will show that:

# this is the file Foo.pm
package Foo;

use strict;
use warnings;

sub new {
    my $class = shift @_;

    print "class = $class\n";

    my $self = {};

    return bless $self, $class;
}

1;

# heres a test program using it, called foo_test.pl
# 

#!/usr/bin/perl

use strict;
use warnings;

use Foo;

use Data::Dumper;

{
    print "Arrow test:\n";
    my $regular_foo = Foo->new();

    print Dumper($regular_foo);

    print "\n\nColon test:\n";

    my $colon_foo = Foo::new();

    print Dumper($colon_foo);

}
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And heres the output from running foo_test.pl

Arrow test:
class = Foo
$VAR1 = bless( {}, 'Foo' );


Colon test:
Use of uninitialized value in concatenation (.) or string at Foo.pm li
+ne 9.
class =
Use of uninitialized value in bless at Foo.pm line 13.
Explicit blessing to '' (assuming package main) at Foo.pm line 13.
$VAR1 = bless( {}, 'main' );
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