Since your teacher wants you to use for, I think the first two tasks are solved OK. But for your own projects, definitely look at the other solutions, too.

Third and fourth task: you wrote:

    if ($numbers[0] > 50) {
        push (@array,$numbers[0]) }
    if ($numbers[1] > 50) {
        push (@array,$numbers[1]) }
    if ($numbers[2] > 50) {
        push (@array,$numbers[2]) }
    if ($numbers[3] > 50) {
        push (@array,$numbers[3]) }
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That's 4 times the (nearly) same statement. And if it were not 4 numbers, but 10 - what then? And earlier, you saw that you can't write

    $sum += 2;
    push(@array,2);
    $sum += 4;
    push(@array,4);
    $sum += 6;
    push(@array,6);
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This is the stage for the other type of "for": either

for (@numbers) {
    push @array, $_ if $_ > 50;
}
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or - for learning purposes more verbose:

for my $current (@numbers) {
    if ($current > 50) {
        push @array, $current;
    }
}
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Of course, this is possible with a C-style for, too, but this one is one of Perl's strengths.

The last task is made for Perl-style for, too.