while( <DATA> ) {
    s[\s(\d+)\d\K\s(?=(\d+)\d\s)]{
        print "$1:$2";
        $1 + 1 == $2 ? "\n" : ' '
    }ge;
    print;
}

##</code><code>##

1051 1061 1071 1081 1091 1101 1111 1121 1131
11151 11161 11171 11181 11191 11201 11211 11221 11231
123451 123461 123471 123481 123491 123501 123511 123531