while(<FILE>) { do stuff }
# rather than
@data = <FILE>;
for (@data) { do stuff }

# pass data to and from subs as a reference (effectively a pointer)
# this saves making duplicate copies of data structures
my $data_ref = process_data(\@data);
sub process_data {
    my $ref = shift;
    for (@$ref) { do stuff }
  return $ref;
}

##</code><code>##

# this is short but memory intensive
do_stuff() if grep { /something/ } @array
# compared to this which is faster and uses less memory
# but takes 4 lines to write instead of one....
for(@array) {
    next unless /something/;
    do_stuff();
  last;
}