use Algorithm::Diff;

say diff_str( qw/xa11211b123 a11222b123x/ );

sub diff_str {
    my $diff = Algorithm::Diff->new( map { [ split // ] } @_ );

    my $diff_str = '';
    while ( $diff->Next ) {
        my ($h1, $h2) = map { join "", $diff->Items( $_ ) } 1, 2;
        $diff_str .= $diff->Diff  ?  "[$h1][$h2]"  :  $h1;
    }
    return $diff_str;
}
#Output: [x][]a112[11][22]b123[][x]