Question

Why do

my $i=0;
my @arr=();

sub readall {
    foreach (@_) {
        $arr[$i] = shift @_;
        $i++;
    }
}

readall(1, 2, 3, 4, 5);
print "@arr"

and

my $i=0;
my @arr=();

sub readall {
    foreach (@_) {
        $arr[$i] = shift @_;
        print $arr[$i];
        $i++;
    }
}

readall(1, 2, 3, 4, 5);

print only three of the arguments to readall?

Why does this function, which seems like it should behave the same, process all five arguments?

sub readall {
    foreach (@_) {
        print $_;
    }
}

readall(1, 2, 3, 4, 5);

This also reads all five (but does operate on a different principle):

my @arr=();

sub readall {
    push(@arr, @_);
}

readall(1, 2, 3, 4, 5);
print "@arr"

Answer 1

Every time you shift your array, it gets shorter… So you are not operating on the whole array, and it will stop early. You can see this by adding a line to your code:

perl -wlae 'my $i=0; my @arr=(); sub readall {foreach (@_) {$arr[$i]=shift @_; $i++; print @_;}} readall(1,2,3,4,5); print "@arr"'

2345
345
45

I assume you can figure it out from here.