use strict;
use warnings;

sub grep_in_place(&\@) {
   my ($cb, $array) = @_;
   my $src = -1;
   my $dst = -1;
   while (++$src < @$array) {
      my $keep;
      $keep = $cb->() for $array->[$src];
      $array->[++$dst] = $array->[$src] if $keep;
   }
   $#$array = $dst;
}

my @a = (1,2,3,4,5,6,7,8);
print("Before: @a\n");
grep_in_place { $_ > 4 } @a;
print("After:  @a\n");