# Initialise an array so that $array[n] = n
my $n = 0;
foreach (@array) {$_ = $n++}
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Putting that aside, an array slice is a good way to do that task without using a loop.

@array[0..100] = 0..100;

# or to mimic the behaviour of your code exactly
@array[0..$#array] = 0..$#array;
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-- Hofmator