There is only one reason to not use your own accessor methods, and that's to microoptimize your code for speed.

Just to see the speed difference between direct vs. method access, I did a little benchmark. If speed is important and you are using accessors repeatedly in a tight loop, there certainly is merit to direct access.

use strict;
use Benchmark "cmpthese";
my $foo = new foo;
cmpthese(-5,{
         direct=>sub{
           $foo->{bar}
         },
         accessor=>sub{
           $foo->bar;
         }
        }
);

package foo;
sub new {
  my $class = shift;
  my $self = {
          bar=>"I am a bar"
         };
  bless $self,$class;
}

sub bar{
  my $self = shift;
  if (@_) {
    return $self->{bar} = shift;
  }
  return $self->{bar};
}
__OUTPUT__

Benchmark: running accessor, direct, each for at least 5 CPU seconds..
+.
  accessor:  6 wallclock secs ( 5.08 usr +  0.01 sys =  5.09 CPU) @ 99
+4059.72/s (n=5059764)
    direct:  4 wallclock secs ( 5.07 usr + -0.01 sys =  5.06 CPU) @ 67
+95643.87/s (n=34385958)
              Rate accessor   direct
accessor  994060/s       --     -85%
direct   6795644/s     584%       --
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