substr($_, $fld1_start, $fld1_len) = substr($_,$fld2_start, $fld2_len);

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

{
  package Foo;

  sub new { 
    my $class = shift;
    return bless( { field => "ABCDEF" }, $class );
  }

  sub field : lvalue { 
    my $self = shift; 
    $self->{'field'};
  }

}

my $f  = Foo->new();

print $f->field, "\n";

$f->field = reverse $f->field;
print $f->field, "\n";

print Dumper $f;

##</code><code>##

ABCDEF
FEDCBA
$VAR1 = bless( {
                 'field' => 'FEDCBA'
               }, 'Foo' );

##</code><code>##

00001AAAAA
00002AAAAA
00003AAAAA
00004AAAAA
00005AAAAA
00006AAAAA
00007AAAAA
00008AAAAA
00009AAAAA
00010AAAAA

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;

use String;
use Data::Dumper;

my $str = String->new();

while (<>) {

  $str->line() = $_;

  print "BEFORE:   ";

  print Dumper $str;

  print "A: ", $str->field_int( "a" ), "\n";
  print "B: ", $str->field_int( "b" ), "\n";

  $str->field_int( "a" ) =  $str->field_int( "b" );

  print "AFTER:   ";

  print Dumper $str;
 
  
  print "LINE: ", $str->line(), "\n" x4;
}

##</code><code>##

package String;

use strict;
use warnings;

sub new {

  my $class = shift;
  my %arg = @_;
  
  my $ddl = { a => { start  => 0,
                     length => 5,
                   },
              b => { start  => 5,
                     length => 5,
                   },
              n => { start  => 10,
                     length => 1,
                   },
            };

  my $self  = { ddl  => $ddl,
                line => undef,
              };

  bless $self, $class;
}

sub line : lvalue {

  my $self = shift;
  my $line = shift;

  $self->{'line'};
}

sub field_start {

  my $self  = shift;
  my $field = shift;

  return $self->{'ddl'}{$field}{'start'};
}

sub field_length {

  my $self  = shift;
  my $field = shift;

  return $self->{'ddl'}{$field}{'length'};
}

sub field_int : lvalue {

  my $self  = shift;
  my $field = shift;

  print "SUBSTR: ", substr($self->{'line'}, $self->field_start($field), $self->field_length($field)), "\n";

  substr($self->{'line'}, $self->field_start($field), $self->field_length($field) );
}

1;

##</code><code>##

BEFORE:   $VAR1 = bless( {
                 'line' => '00001AAAAA
',
                 'ddl' => {
                            'n' => {
                                     'length' => 1,
                                     'start' => 10
                                   },
                            'a' => {
                                     'length' => 5,
                                     'start' => 0
                                   },
                            'b' => {
                                     'length' => 5,
                                     'start' => 5
                                   }
                          }
               }, 'String' );
SUBSTR: 00001
A: 00001
SUBSTR: AAAAA
B: AAAAA
SUBSTR: AAAAA
SUBSTR: 00001
AFTER:   $VAR1 = bless( {
                 'line' => 'AAAAAAAAAA
',
                 'ddl' => {
                            'n' => {
                                     'length' => 1,
                                     'start' => 10
                                   },
                            'a' => {
                                     'length' => 5,
                                     'start' => 0
                                   },
                            'b' => {
                                     'length' => 5,
                                     'start' => 5
                                   }
                          }
               }, 'String' );
LINE: AAAAAAAAAA

BEFORE:   $VAR1 = bless( {
                 'line' => '00002AAAAA
',
                 'ddl' => {
                            'n' => {
                                     'length' => 1,
                                     'start' => 10
                                   },
                            'a' => {
                                     'length' => 5,
                                     'start' => 0
                                   },
                            'b' => {
                                     'length' => 5,
                                     'start' => 5
                                   }
                          }
               }, 'String' );
SUBSTR: 00002
A: 00002
SUBSTR: AAAAA
B: 00002
SUBSTR: AAAAA
SUBSTR: 00002
AFTER:   $VAR1 = bless( {
                 'line' => '00002AAAAA
',
                 'ddl' => {
                            'n' => {
                                     'length' => 1,
                                     'start' => 10
                                   },
                            'a' => {
                                     'length' => 5,
                                     'start' => 0
                                   },
                            'b' => {
                                     'length' => 5,
                                     'start' => 5
                                   }
                          }
               }, 'String' );
LINE: 00002AAAAA

etc.