until is a looping construct. The entire loop is this:

push @group, $entire_file[$i] until ( $i % 5 == 0 );
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Which can also be written as this:

while( $i % 5 != 0 ) {
  push @group, $entire_file[$i];
}
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As you can see (or should be able to see if you've spent any time at all programming), $i never changes in this loop. ...so it will either not execute at all (if $i is a multiple of 5), or will execute forever otherwise. Well, "forever", until you fill your memory with copies of $entire_file[$i].

You might have intended something more along the line of:

push @group, $entire_file[$i] unless $i % 5 == 0;
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That's not a looping construct -- it's just a conditional. In this way, $i will have its value change as the outer foreach loop progresses.