my $graffito = 'Richard was here.';
substr($graffito, 8, 3) = 'is'; # now, it's "Richard is here."

##</code><code>##

rdice@tanru:~$ cat lvalue2.pl && ./lvalue2.pl
#!/usr/bin/perl -w

######################################################################
#
#  lvalue2.pl
#             Another quick, dirty and simple program meant to
#             illustrate the creation and use of lvalue subroutines.
#             This is part of my "What Fresh L is this?" Lightening
#             Talk for the Toronto Perl Mongers, Thu 26 Sept 2002.
#
#  Author:  Richard Dice (rdice@pobox.com)
#  Date:    Tue 24 Sept 2002
#
######################################################################

use strict;   # Always use strict.  ALWAYS USE STRICT.

my $graffito;

$graffito = 'Richard was here';
print ((substr($graffito, 8, 3) = 'is'), "\n");   # to see return value
print "$graffito\n";                              # to see end result

print "\n";                                       # whitespace between outputs

$graffito = 'Richard was here';
print ((mysubstr($graffito, 8, 3) = 'is'), "\n"); # to see return value
print "$graffito\n";                              # to see end result

# Were 'mysubstr' to work correctly, then the two outputs should be
# identical

exit 0;

sub mysubstr : lvalue {

    my @chars = split //, $_[0];

    my $pos   = $_[1];
    my $width = $_[2];

    # Wouldn't it be nice if the rvalue was added onto the end of @_?
    # As it stands, I have no idea how to get at the rvalue.  I'll
    # just pretend that this is how it should be handled for the sake
    # of this example.

    my $rvalue = $_[3];

    # For all you pedants out there, I know that my algorithm for
    # putting the string back together doesn't exactly equal what
    # the real 'substr' function would at its edge cases.
    # That's not the point of this example.

    $_[0] = (join '', @chars[0..$pos-1])
      . $rvalue . (join '',@chars[$pos + $width .. $#chars]);

    $rvalue;  # This seems like the easiest way to return what the
              # real 'substr' returns without mucking anything up.
}
is
Richard is here

Use of uninitialized value in concatenation (.) or string at ./lvalue2.pl line
54.
is
Richard  here
rdice@tanru:~$