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# ImgSum.pm
package ImgSum;
...
}

1;

#!/usr/bin/perl

...
store($sum, $out);

# SumDif.pm
package SumDiff;
...
}

1;

#!/usr/bin/perl

...
    }
}

( take a picture of the empty room, save it as ref0.jpg )
...

$ ./watch.pl