Try this as a variation:

my $x = 0;
print "$x", $x++;
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and figure out why this does print "00".

The reason for the difference is that on your case the first parameter is an alias to the variable $x, while in my case it's an expression with a stringified copy of the value of $x at the time the parameter value was evaluated.

An alias can still change its value later – when the value of $x changes. A copy can not.