my $critical_data = 'Original Value';

print "Begin: $critical_data\n";
SubOne($critical_data);
print "End: $critical_data\n";


sub SubOne {
    print "\tSubOne Got: @_\n";
    SubTwo();
}

sub SubTwo {
    print "\tSubTwo Got: @_\n";
    $_[0] = 'pwned!';
}
__END__
Begin: Original Value
        SubOne Got: Original Value
        SubTwo Got:
End: Original Value