use strict;
use warnings;

#                        1         2
#              012345678901234567890123456789
my $string = q(The cat scattered caterpillars\n);
#                  ^    ^        ^
#                  4    9        18

print "\n Match against string - $string\n";

print "Start matching\n\n";
my $match = 1;
while($string =~ /(cat)/g)
{
    print "Match @{[$match ++]}\n";
    print "                found - $1\n";
    print "       Value of \$-[1] - $-[1]\n";
    print "       Value of \$+[1] - $+[1]\n";
    print "       Value of \$+[0] - $+[0]\n";
    print "         Value of \$#+ - $#+\n";
    print "Value of pos(\$string) - @{[pos($string)]}\n";
}

print "\nStart annotation\n\n";

$string =~ s
   {
      (cat)(?{print "Value of pos(\$string) - @{[pos($string)]}\n"})
   }
   {
      $1 . "[@{[pos($string)]}]"
   }xeg;

print "\n     Annotated string - $string\n";

##</code><code>##

 Match against string - The cat scattered caterpillars

Start matching

Match 1
                found - cat
       Value of $-[1] - 4
       Value of $+[1] - 7
       Value of $+[0] - 7
         Value of $#+ - 1
Value of pos($string) - 7
Match 2
                found - cat
       Value of $-[1] - 9
       Value of $+[1] - 12
       Value of $+[0] - 12
         Value of $#+ - 1
Value of pos($string) - 12
Match 3
                found - cat
       Value of $-[1] - 18
       Value of $+[1] - 21
       Value of $+[0] - 21
         Value of $#+ - 1
Value of pos($string) - 21

Start annotation

Value of pos($string) - 7
Value of pos($string) - 12
Value of pos($string) - 21

     Annotated string - The cat[4] scat[9]tered cat[18]erpillars