The BEGIN block can see $no_go because lexical scoping is done at compile-time whereas the initialisation of the variables isn't done 'til run-time e.g

use strict;
my $foo = "defined";

BEGIN {
    print "foo is ", defined($foo) ? $foo : 'undef',
          " during BEGIN phase\n";
};

print "foo is ", defined($foo) ? $foo : undef, " at runtime\n";

__output__

foo is undef during BEGIN phase
foo is defined at runtime
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See. the my variables section of Lexical scoping like a fox for more details.
HTH

_________
broquaint