A alternate solution, just for fun.

use strict;
use warnings;

my $paren_grp_re  = qr/ \( [^  ) ]* \) /x;
my $square_grp_re = qr/ \[ [^ \] ]* \] /x;
my $curly_grp_re  = qr/  { [^  } ]*  } /x;
my $angle_grp_re  = qr/  < [^  > ]*  > /x;

my $string = '<x1>[x2]{x3}(x4)';

print "Before: $string\n";

$string = join '+',
          map / $paren_grp_re
              | $square_grp_re
              | $curly_grp_re
              | $angle_grp_re
              | [^\(\[\{\<]
              /xg,
          $string;

print "After: $string\n";
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Doesn't support nesting.