With the way that Perl OO magic works, when you do

my $o = Some::Class->new;
$o->open;
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the $o->open turns into Some::Class::open($o) - by calling it as a method, you're effectively prefixing a namespace to it, so it will never collide with anything outside that namespace and is, therefore, safe. (Unless your class is named 'main', I suppose, but you really, really shouldn't be doing that anyhow.)