#!perl
use strict;
use warnings;
$|++;
my ($comp,$first,@sec);

$comp = "descr is ".(defined $first ? q{$first} : 'null').' and list is '.( @sec ? q{join ' ', @sec} : 'empty');
print "Oringinal:$comp\n";
&modify_first('UNO');
&modify_second (1);&modify_second (2);
&modify_first ('DUE');
&reset_second;


sub modify_first{$first = shift;    print "$comp\n";}
sub modify_second {push @sec, shift; print "$comp\n";}
sub reset_second {@sec=qw();        print "$comp\n";}

__OUTPUT__
Oringinal:descr is null and list is empty
descr is null and list is empty
descr is null and list is empty
descr is null and list is empty
descr is null and list is empty
descr is null and list is empty

##</code><code>##

my $text = q{
  Dear $person,
  I know that this text is $adjective.
  But I wish it could be...

};

my $person    = 'Mom';
my $adjective = 'not interpolated';

print eval "qq{$text}";

__OUTPUT__

  Dear Mom,
  I know that this text is not interpolated.
  But I wish it could be...


##</code><code>##

my $n = sub {1};
my $m = sub {&$n*2};
my $o = sub {&$m*2};
my $s = sub {&$n." ".&$m." ".&$o."\n"};
print &$s;
$n = sub {2};
print &$s;

__OUTPUT__
1 2 4
2 4 8

##</code><code>##

  How can I expand variables in text strings?
    Let's assume that you have a string that contains placeholder variables.

        $text = 'this has a $foo in it and a $bar';

    You can use a substitution with a double evaluation. The first /e turns
    $1 into $foo, and the second /e turns $foo into its value. You may want
    to wrap this in an "eval": if you try to get the value of an undeclared
    variable while running under "use strict", you get a fatal error.

        eval { $text =~ s/(\$\w+)/$1/eeg };
        die if $@;

    It's probably better in the general case to treat those variables as
    entries in some special hash. For example:

        %user_defs = (
            foo  => 23,
            bar  => 19,
        );
        $text =~ s/\$(\w+)/$user_defs{$1}/g;


##</code><code>##

my $color = "red";
my $fruit = "apple";
my $name = "chromatic";

my $string = 'Hi, my name is $name.  Please hand me a $color $fruit.';
print ">>$string<<\n";    # demonstrate what we have

my $s2;
eval "\$s2 = qq/$string/";    # the real magic
print "->$s2<-\n";    # demonstrate the result

__OUTPUT__
>>Hi, my name is $name.  Please hand me a $color $fruit.<<
->Hi, my name is chromatic.  Please hand me a red apple.<-