Because it allows me to 'hardcode' the numbers in the substrs.

If you uncomment the print before the eval, you'll see something like:

package Test;
use constant { F1_N => 0, F2_N => 15, F3_N => 30, F4_N => 45, F5_N => 
+60, F6_N => 75, F7_N => 90, F8_N => 105, F9_N => 120, F10_N => 135, }
+;
use constant { F1_L => 15, F2_L => 15, F3_L => 15, F4_L => 15, F5_L =>
+ 15, F6_L => 15, F7_L => 15, F8_L => 15, F9_L => 15, F10_L => 15, };

sub new {
    my $class = shift;
    my $self = shift // '';
    return bless \$self, $class
}
# line 1 "sub_F4"
sub Test::F4 :lvalue {
    my $self = shift;
    substr( $$self, F4_N(), F4_L() );
}
[download]

F4_N() and F4_L() are constant subs which get optimised away during compilation, leaving hardcoded numbers which are faster than variables.

The memory saving comes from packing the fields into single strings; the performance comes from asking the sub to do as little as possible.

That said, by explaining that, I've spotted another couple of optimisations; and a potential bug. I'll get back to you with a revised version 2 days from now.

(A good reason for not uploading to cpan straight away :)