It is the OO Perl. The first argument is the invocant, the second argument is the interval (but according to the documentation, might as well be an array ref). The rest of the parameters are left in @_ to be used later.

Handling the array ref:

if (ref $millis eq 'ARRAY') {
        $callback = Tk::Callback->new($millis->[1]);
        $millis = $millis->[0];
    }
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Handling the remaining variable references:

foreach my $vref (@_) {
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