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use strict;
use warnings;
...
    print STDOUT "here is dollar-underscore: $_\n";
    }
}

test of username: adad
test of dollar-underscore: ADELMAN,John,adad,Ray
...
Use of uninitialized value $username in concatenation (.) or string at
+ test.pl line 14, <> line 4.
here is the username: 
here is dollar-underscore: