$ perl -Mstrict -Mwarnings -le '
    package myDebugger;
    {
        my $debugger;
        sub GetInstance { defined $debugger ? $debugger : DebuggerInit() }
        sub DebuggerInit { $debugger = "singleton" }
    }
    # a typo later in your code (perhaps "$debug = 0;" was intended)
    my $debug = 1;
    # ...
    $debugger = 0;

    package main;
    print myDebugger::GetInstance();
    print myDebugger::GetInstance();
'
Global symbol "$debugger" requires explicit package name at -e line 11.
Execution of -e aborted due to compilation errors.