The interpreter is behaving as if it is evaluating $x++ before it prints anything out. The initial value of $x is 0. So, when $x++ is evaluated, $x then becomes 1.

Try this:

print "x="   . $x   . "\n";
print "x++=" . $x++ . "\n";
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You should get this output, which is more like what you expect.

x=0
x++=0
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