Look up "Tk::fileevent":

       Tk::fileevent - Execute a callback when a filehandle
       becomes readable or writable

SYNOPSIS
       $widget->fileevent(fileHandle,readable?,callback?)

       $widget->fileevent(fileHandle,writable?,callback?)

DESCRIPTION
       This command is used to create file event handlers.  A
       file event handler is a binding between a filehandle and a
       callback, such that the callback is evaluated whenever the
       filehandle becomes readable or writable.  File event han­
       dlers are most commonly used to allow data to be received
       from another process on an event-driven basis, so that the
       receiver can continue to interact with the user while
       waiting for the data to arrive...
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