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use warnings;
use strict;
...

say "the sum of 9 and $b is ", &sum_and_diff(9, $b, \$diff);
say "and the difference is ", $diff;  # this is line 34

The address of $varible, which contains the value 22,
is SCALAR(0x801e64540)
...
the sum of 9 and 2 is 11
Use of uninitialized value $diff in say at line 34.
and the difference is