The parent assumed the accumulator was initially zero. That's not likely in a real-life scenario, so this removes the assumption: 
my @stack;
my $acc = rand;

sub PUSH() { push(@stack, $acc); }
sub POP()  { pop(@stack);        }
sub INC()  { $acc++;             }
sub ADD()  { $acc += POP;        }
sub SUB()  { $acc -= POP;        }

PUSH;
SUB;
INC;
INC;
PUSH;
ADD;

print("$acc\n");

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In reply to Re^2: Adding 2 + 2 (stack machine,bug fix) by ikegami
in thread Adding 2 + 2 by merlyn

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