#!/usr/bin/perl

use strict;
use warnings;

my $string = "ABC[GHI]XYZ";
my @array;

splitstring($string);

push @array, split//, $string;

print "$_ " for sort @array;

sub splitstring
{
    return unless ( $string =~ /.*\[\w+\].*/ );
    if ( $string =~ /.*(\[(\w+)\]).*/ )
    {
        my $remove = $1;
        my $str    = $2;
        print "rem = $remove\n str = $str\n";
        push @array, $str;
        $string =~ s/\Q$remove\E//;
    }
    splitstring($string);
}