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my ($idx, @array, $a_big_num, $some_data);

...
# Fetch the data
$idx = map_func($a_big_num);
$some_data = $array[$idx];

my (%hash, $a_big_num, $some_data);

...

# Fetch
$some_data = $hash{$a_big_num};

sub map_func { return 7 }

sub map_func {
   state %indexes;
...
   die "Ran out of slots!" if keys %indexes > 1000;
   return $indexes{$a_big_num}=$idx++;
}