I don't know how you made that determination but I think something else is happening. The following code shows that the string only gets evaled with the ref itself is called.

use strict;
use warnings;

my $code = 'print "Hello\n"';

print "BEFORE\n";
my $test;
$test->{a} = sub { eval $code};
print "BETWEEN\n";
$test->{a}->();
print "AFTER\n";
[download]

Outputs

C:\Perl\test>perl order_sub.pl
BEFORE
BETWEEN
Hello
AFTER
[download]

You can see the hello is only printed with the ref is executed and not when it is defined.