use strict;
use warnings;

use re q{eval};

sub doSep { print q{-} x 40, qq{\n}; }

my $count     = 0;
my $string    = q{<x1>[x2]{x3}(x4)};
my $rxClose   = qr@[]>})]@;
my $rxOpen    = qr@[[<{(]@;
my $rxBetween = qr
   {(?x)
       (?<=($rxClose))
       (?=($rxOpen))
       (?{print qq{Match @{ [++ $count] }: left $1, right $2\n}})
   };

print qq{ Before: $string\n};
doSep();

$string =~ s{$rxBetween}{+}g;
doSep();

print qq{  After: $string\n};

##</code><code>##

 Before: <x1>[x2]{x3}(x4)
----------------------------------------
Match 1: left >, right [
Match 2: left >, right [
Match 3: left ], right {
Match 4: left ], right {
Match 5: left }, right (
Match 6: left }, right (
----------------------------------------
  After: <x1>+[x2]+{x3}+(x4)

##</code><code>##

use strict;
use warnings;

use re q{eval};

sub doSep { print q{-} x 40, qq{\n}; }

my $count     = 0;
my $string    = q{<x1>[x2]{x3}(x4)};
my $rxClose   = qr@[]>})]@;
my $rxOpen    = qr@[[<{(]@;
my $rxBetween = qr
   {(?x)
       ($rxClose)
       ($rxOpen)
       (?{print qq{Match @{ [++ $count] }: left $1, right $2\n}})
   };

print qq{ Before: $string\n};
doSep();

$string =~ s{$rxBetween}{$1+$2}g;
doSep();

print qq{  After: $string\n};

##</code><code>##

 Before: <x1>[x2]{x3}(x4)
----------------------------------------
Match 1: left >, right [
Match 2: left ], right {
Match 3: left }, right (
----------------------------------------
  After: <x1>+[x2]+{x3}+(x4)