You are passing in $booking_dlg to Wx::Panel::new, which is presumably creating a Wx::Panel object, which recursively refers to at least 3 other objects. If any of these other objects saves the reference to $booking_dlg, then you have a circularity of the form:

$booking_dlg->{booking_main} == Wx::Panel object,
(Wx::Panel object)->{foo} == other object
(other object)->{bar} = $booking_dlg
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which will preserve the Wx::Panel object, the other object, and the $booking_dlg object forever unless ->{booking_main} gets reassigned sometime later. (and if $booking_dlg is simply being tossed later on, there's your leak right there)

(Update:  Well, okay, it could also be (Wx::Panel object)->{foo} = $booking_dlg directly and the other 3 objects are just being dragged along for the ride. It only takes one such reference to ruin your day...)