I use $#data ++ because I just followed some example I found.

Dangerous approach. You might get bitten if you use code you don't understand. And this example particularly is very obscure - please avoid that source of examples.

Let's take a close look at this piece of code:

$data[ $#data ++ ] = $1;
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What happens here, and why does it work, but not as expected?

Consider:

• An empty array has no elements.
• The index -1 references the last element of an array.
• $#data is the index of the last element of @data.
  • If @data has 1 element, it is 0.
  • If @data has no elements, it is -1.
• Assigning to the last element of an empty array (which doesn't exist) gives a runtime error:
  Modification of non-creatable array value attempted, subscript -1 at ...
• Array elements can be pre-allocated assigning to $#data.
• $#data ++ is a post-increment.

Putting the above pieces together, the following happens the first time through the loop:
The last element of the empty array @data is requested by evaluating $#data, which yields -1. Finishing the evaluation of the term inside brackets [ $#data ++ ], the value of $#data is incremented after establishing the index requested, which leads to the creation of the first array element which index 0, so accessing the last element (index -1) is valid, and $1 is assigned to the first slot (which happens to be the last) at index 0.

The next time through the loop, $#data yields 0, after that it is incremented, creating a slot at index 1. The assignment is also to the element at index 0, which means its contents from the previous assignment is overwritten.

After the loop, @data has a dangling empty array element, and the first value is gone. Had you used warnings, you would have seen that:

use warnings;
my @data;
for( 1..3 ) {
    $data[ $#data ++ ] = $_;
}
for (@data) {
    print $_,"\n";
}
__END__
2
3
Use of uninitialized value $_ in print at - line 7.
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Perl tries hard to do what you mean... if you know what you mean.

Changing the post-increment to a pre-increment yields the desired effect:

use warnings;
my @data;
for( 1..3 ) {
    $data[ ++ $#data ] = $_;
}
for (@data) {
    print $_,"\n";
}
__END__
1
2
3
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- but it is unwieldy, and as choroba above says, better use push which is the preferred idiom.