Fastest and smallest are incompatible.

Also do you need at least 30, or exactly?

If I wanted to be bare bones I would do something like this untested code:

use strict;
use vars qw($max_size $reset_size);
$max_size = 60;
$reset_size = 30;
{
  my %count;
  my %cache;

  sub get_elem {
    my $id = shift;
    if (exists $count{$id}) {
      ++$count{$id};
      return $cache{$id};
    }
    else{
      return;
    }
  }

  my $size;
  sub set_elem {
    {
      my $id = shift;
      my $elem = shift;
      if (exists $count{$id}) {
        $cache{$id} = $elem;
      }
      else {
        if ($max_size < ++$size) {
          _clear_cache();
        }
        $count{$id} = 0;
        $cache{$id} = $elem;
      }
      redo if @_;
    }
  }

  sub _clear_cache {
    my @ids = sort {$count{$b} <=> $count{$a}} keys %count;
    @ids = @ids[0..($reset_size-1)];
    my %new;
    %count = ();
    foreach (@ids) {
      $new{$_} = $cache{$_};
      $count{$_} = 0;
    }
    %cache = %new;
    $size = $reset_size;
  }
}
[download]

Note that I go over 30 here, but doing so makes the actual lookup much faster.

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