A couple lines fixed...

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11124042
use warnings;
use v5.10;

my $variable = 22;
my $pointer = \$variable;

say "The address of \$varible, which contains the value $variable,";
say "is $pointer";

$$pointer = 25;

say "Look at that!  \$variable now equals $variable";

sub sum_and_diff {
    my $a = shift @_;
    my $b = shift @_;
#    my $res = \(shift @_); # why does the "\" work here? # it didn't
    my $res = shift @_;
    my $sum = $a + $b;
    $$res = $a - $b;
    return $sum;
}

my $b = 2;
my $diff;  # this is line 27
my $pointer_to_diff = \$diff;

say "the sum of 5 and $b is ", &sum_and_diff(5, $b, $pointer_to_diff);
#say "and the difference is ", $pointer_to_diff;
say "and the difference is ", $$pointer_to_diff;

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

The address of $varible, which contains the value 22,
is SCALAR(0x55d283691438)
Look at that!  $variable now equals 25
the sum of 5 and 2 is 7
and the difference is 3
the sum of 9 and 2 is 11
and the difference is 7
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