Here is a much simpler case:

package DEF;
sub new { bless [], shift }

package ABC;
sub new { bless {}, shift }
sub DEF { "ABC's DEF function" }

# still in package ABC!
$x = new ABC;
$y = new DEF;
$z = new DEF::;
print "X=$x\nY=$y\nZ=$z\n";

__END__
X=ABC=HASH(0x8151d6c)
Y=ABC's DEF function=HASH(0x8151b44)
Z=DEF=ARRAY(0x81525c4)
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The problem is that, while in the ABC package, Perl sees new DEF as two function calls, not as an indirect object call. This is simply because of the order that Perl tries things in -- since it knows 'new' and 'DEF' are declared functions at that point, that's how it treats both of them. By using new DEF::, we force Perl to interpret it as an object call. Sadly, not even DEF->new will work, because Perl will try calling the "new" method of the package name returned by the DEF() function, "ABC's DEF function"!

Long story short, this is just how Perl parses your code. You shouldn't give functions and packages identical names. (Indirect object notation is smelly and should be avoided anyway.) To be foolproof, always append "::" to the end of your class names. DEF::->new and new DEF:: both behave properly in this case.