\&get_callback doesn't call get_callback; it returns a reference to it. This reference will be used by get to invoke the sub at a later time (e.g. when the request succeeds or fails). When get invokes it, it will pass parameters to it. 

sub get {
   my ($ua, $url, $cb) = @_;
   ...
   $cb->($foo, $tx);  # Call the referenced sub.
   ...
}

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In reply to Re: Trying to Understand Callback(s) by ikegami
in thread Trying to Understand Callback(s) by DanielSpaniel

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