gistfile1.txt

open my $fh, '<', $big_file or die;
for my $line ( <$fh> ) {    # this reads the whole file into memory
   # ...
}



vs.

open my $fh, '<', $big_file or die;
while ( my $line = <$fh> ) {   # reads file line by line, uses less memory
    # ...
}





my @big_array = ('all','data','in','one','big','array');
my @filtered  = grep{ $_ =~ m/a/ }@big_array;
# now two big arrays in memory
my @second_level = map{ $_ . " " . $_ }@filtered;
# now three big arrays in memory

vs.

my @filter;
{
    my @big_array  = ('all','data','in','one','big','array');
    @filtered = grep{ $_ =~ m/a/ }@big_array;
}
my @second_level = map{ $_ . " " . $_ }@filtered;
# Perl now reuses memory as @big_array is out of scope in memory "freed" internally