This is a fun little TIMTOWTDI question :-) Here's another solution:

my @array = (1, 2, 3, 4, 2, 1, 2, 0, 1, 0, 0);

@array = do { my @tmp = grep $_, @array; ((0)x(@array-@tmp),@tmp) };

print "@array\n";  # prints "0 0 0 1 2 3 4 2 1 2 1"
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