trace_var(...) public

Controls tracing of assignments to global variables. The parameter +symbol_ identifies the variable (as either a string name or a symbol identifier). cmd (which may be a string or a Proc object) or block is executed whenever the variable is assigned. The block or Proc object receives the variable’s new value as a parameter. Also see Kernel::untrace_var.

   trace_var :$_, proc {|v| puts "$_ is now '#{v}'" }
   $_ = "hello"
   $_ = ' there'


   $_ is now 'hello'
   $_ is now ' there'
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